For the Blackberry Mousse Filling
Make the blackberry mousse first so it has time to chill. Or better yet, make this mousse in advance! It can be made 1-2 days ahead of time.
Add ¾ cup white chocolate chips and ¼ cup heavy cream in a medium-sized, heat-proof bowl. Heat for 45-60 second in the microwave, then let the mixture sit for a minute. Stir until combined with a rubber spatula. The white chocolate should be completely melted, and the mixture should be smooth. If you don’t have a microwave, you can also use a double boiler.
Stir 1/3 cup of blackberry jam into the white chocolate mixture. If desired, add a small squirt of purple gel food coloring to give this mousse a more vibrant purple color. Mix thoroughly until the mixture is smooth and there are no clumps of jam. Set aside.
Add ¾ cup of heavy cream, 1/3 cup powdered sugar, 1 teaspoon vanilla extract, and 1/8 teaspoon fine salt into a large bowl or the bowl of a stand mixer fit with a whisk attachment.
Mix on a medium-high speed until the whipped cream starts to gather in the whisk attachment and you can see texture around the edge of the bowl. The whipped cream should be thick and able to keep its shape. This usually takes about a minute or two in my stand mixer.
Gently fold half of the whipped cream into the blackberry white chocolate mixture using a rubber spatula. Use the spatula to swoop around the sides of the bowl and pull through the center. Fold in the remaining whipped cream. Be careful not to over-mix the mousse, or it will become deflated and lose its structure.
Cover the mousse with plastic wrap and chill in the refrigerator for at least 2 hours. Make sure you keep the mousse chilled until right before you fill the cake because it can lose its shape if it gets warm.
For the Blackberry Swirl Cake
Next, make the blackberry swirled cake layers! Preheat the oven to 350°F / 175°C. Line and grease three, 8-inch cake pans with parchment paper and nonstick baking spray or homemade cake release. If you don’t have 3 cake pans, the batter can sit at room temperature for a few hours. Bake as many layers at a time as you can at a time.
Add 1 cup of unsalted butter and 2 ½ cups of granulated sugar into the bowl of a stand mixer. Mix together on a high speed with a whisk attachment for 3 minutes. Scrape down the sides of the bowl as needed with a rubber spatula. The mixture should become lighter in color as air is whipped into it.
Add in 1 cup of room temperature egg whites and mix at a medium speed until they’re incorporated.
Whisk 3 cups of cake flour, 2 ½ teaspoon baking powder, and 1 teaspoon salt together in a separate bowl.
Add half of the dry ingredients into the butter/egg mixture and mix on a low speed until combined.
Add in 1 ½ cups sour cream, ¼ cup oil, and 1 teaspoon vanilla extract and mix at a low speed until combined. Scrape the sides and bottom of the bowl as needed.
Mix in the remaining dry ingredients on a low speed.
Pour half of the batter evenly between the prepared cake pans. I added about 275g of batter per pan.
Drizzle ½ cup of blackberry jam over the cake batter. I put my jam in a piping bag to make it easier to drizzle, but you can also use a spoon. Use a small offset spatula or butter knife to swirl the blackberry jam into the cake batter.
Top with the remaining batter, adding another 275g of batter to each pan. Smooth the batter with a spatula or small offset spatula so that it’s even across the pan.
Bake for 32-35 minutes, or until a toothpick comes out with a few moist crumbs. Rotate the pans halfway through to help them bake evenly. Note that this recipe doesn’t brown as much as my vanilla layer cake recipe, so be sure to test the layers with a toothpick rather than rely on visual browning cues!
Let the pans cool for 10 minutes, then run an offset spatula around the perimeter of the pan to separate the cake from the pan.
Place the cake layers into the freezer for 45 minutes to accelerate the cooling process. Once the layers are fully cooled, carefully flip the pans, and remove the layers from the pans.
Use a serrated knife to level the top of each cake layer. If you make these in advance, wrap and freeze them at this point.
For the Blackberry Buttercream Frosting
While the cake layers bake and cool, make the blackberry buttercream frosting.
Beat 2 cups butter on a low speed for 30 seconds with a paddle or whisk attachment until smooth.
Add in 1 tablespoon vanilla extract and ½ teaspoon salt and beat on a low speed.
Gradually mix in 6 cups of powdered sugar and 1 tablespoon of lemon juice on a low speed. Continue to mix until the ingredients are fully incorporated.
Scoop 1/3 cup of white frosting into a small piping bag fit with a small French piping tip and seal the top of the bag with a clip or rubber band. Set aside.
Mix ½ cup of blackberry into the remaining frosting on a low speed until combined and the desired consistency is reached. If the frosting seems too thick, add in additional cream (1 teaspoon at a time). If the frosting is too thin, add in more powdered sugar (quarter of a cup at a time).
Place 1 cup of the frosting in a separate bowl and color it a pink-purple color by adding in a squirt of pink gel food coloring. Place this frosting in a small piping bag fit with an open star piping tip (like a Wilton 1M) and set aside.
Add a squirt of purple gel food coloring into the remaining frosting and stir until it’s evenly colored. Place the purple frosting in a large piping bag. Seal the top of the bag with a rubber band or clip and cut an opening that’s about 3/4-inch wide at the base of the bag. Set aside.
Assembling the Blackberry Mousse Cake
Stack and frost the cake layers on a greaseproof cake board using a dab of frosting to help stick the first cake layer to the board.
Pipe a double ring of purple blackberry buttercream around the perimeter of the cake layer. Spread ¼ cup of blackberry jam inside the frosting ring, then top it with an even layer of blackberry mousse. Use a little less than ½ of the mousse between each layer (you will end up having a tiny bit leftover). Gently press blackberries into the mousse.
Chill the cake in the freezer for about 10 minutes, then repeat with the next cake layer. This helps lock the filling in place and prevents the layers from sliding around as you frost the cake. Once all the cake layers are stacked, spread a thin coat of frosting around the cake to fully cover the cake layers. If your layers are sliding around, insert a chopstick or wooden dowel into the center of the cake to make it more stable.
Smooth using a bench scraper, then chill the cake in the fridge (30 minutes) or freezer (10 minutes) until the frosting is firm to the touch.
Add a second, thicker layer of purple blackberry buttercream to the cake and smooth using a bench scraper.
Color any remaining purple buttercream a deeper shade of purple by adding a squirt of purple gel food coloring. Place this frosting in a small piping bag fit with a small French piping tip and seal the top of the bag with a clip or rubber band.
Pipe swirls of the different colors of buttercream in a curved line around the side of the cake and up on top of the cake like in the pictures above. Start with the pink buttercream and pipe large buttercream rosettes, then gently press some fresh blackberries into place. Fill in the gaps with swirls and dollops of the other colors of buttercream, then enjoy!
Tips for Making the Best Blackberry Mousse Cake
- When making the blackberry mousse, chill your mixing bowl and whisk attachment in the freezer. It makes it easier and quicker to whip up the whipped cream for the mousse.
- Properly measure your flour (spoon into the cup measure, then level). Or better yet, use a kitchen scale to measure your dry ingredients.
- Mix your cake batter just until the ingredients are incorporated after adding in the flour. This ensures your cake layers are tender and fluffy.
- Use a scale to weigh your cake pans as you fill them. This will make your cake layers bake to the same height and bake more evenly.
- Bang your cake pans on the counter before putting them in the oven. This brings any air bubbles that are trapped in the batter to the surface.
- Level your room temperature or thawed cake layers with a serrated knife to make them easier to stack.
- Chill the leveled cake layers in the freezer for about 20 minutes before assembling the cake. It makes it so much easier to frost them!
- Make your cake layers in advance and freeze them. It breaks the process up and makes it more approachable.
- Make your frosting ahead of time too or save any leftover frosting! It can be stored in an airtight container in the fridge for up to a month, or in the freezer for up to 3 months. Be sure to give it a good stir once it thaws to get the consistency nice and smooth again.
- The blackberry mousse filling in this cake can be made up to two days in advance if stored in an airtight container in the fridge.
- A frosted cake can last in the fridge for up to 2 days.