For the Cinnamon Cake
Preheat oven to 350°F. Grease 3, 8-inch cake pans and line them with parchment rounds.
Add the unsalted butter and granulated sugar into the bowl of a stand mixer. Beat on a high speed with a whisk attachment for a few minutes, scraping down the sides of the bowl as needed with a rubber spatula. The mixture will become lighter in color as air is whipped into it.
Add the egg whites and mix at a medium speed until they’re incorporated.
Sift the dry ingredients (cake flour, baking powder, ground cinnamon and salt) into a separate bowl. Add half of the dry ingredients into the butter mixture, and mix on a low speed until just incorporated.
Add the sour cream, vanilla extract, and oil, and mix at a low speed until combined, scraping the sides and bottom of the bowl as needed.
Mix in the remaining dry ingredients on a low speed.
Pour the batter evenly between the prepared cake pans. Chelsey likes to use a digital kitchen scale to weigh her pans, to make sure the layers will be the same height.
Bake for 31-34 minutes, or until a toothpick comes out with a few moist crumbs. Rotate pans halfway through to help them bake evenly.
Allow the pans to cool for 10 minutes, then run an offset spatula around perimeter of the pan to separate the cake from the pan and flip the layers onto a wire rack to finish cooling.
Once the cake layers have fully cooled, use a serrated knife to level the tops and then cut each cake layer in half horizontally, so that you have six thin cake layers.
For the Blackberry Peach Buttercream Frosting
While the cake layers bake and cool, make the blackberry peach buttercream frosting.
Beat the butter on a medium speed for 30 seconds with a hand mixer or stand mixer fit with a paddle attachment until smooth.
Mix in the vanilla extract and salt on a low speed until incorporated.
Slowly add in the powdered sugar. Add in the heavy cream half way through to make the frosting easier to mix.
Continue to mix until the ingredients are fully incorporated, and the desired consistency is reached. Place 1/2 cup of the plain vanilla buttercream into a small piping bag fit with a small French tip and set aside – this will be used to decorate the sides and top of the cake later in the process.
Mix the blackberry jam and peach preserves into the remaining buttercream.
If the frosting is too thick, add in additional cream (1 Tbsp at a time). If the frosting is too thin, add in more powdered sugar (¼ cup at a time).
Stir by hand with a rubber spatula to make the frosting extra smooth. Place 1 cup of frosting in a small piping bag fit with a round piping tip, then cover the remaining frosting with plastic wrap and set aside.
For the Assembly
Stack and frost cake layers on a greaseproof cake board, using a dab of frosting to help stick the first cake layer to the board.
Spread an even layer of blackberry peach buttercream on top of the first cake layer. Pipe a ring around the cake layer to act as a frosting dam to keep the filling in place.
Carefully spread 2 Tbsp of the blackberry jam on top of the frosting and top with a 1/3 cup of fresh blackberries.
Repeat this process with the 2nd cake layer, but use 2 Tbsp of peach preserves and top with 1/3 cup of diced peaches.
Chill the cake in the freezer for 5 minutes once the two layers are stacked to help lock the filling in place and make it easier to stack the remaining cake layers.
Repeat this process with the remaining cake layers.
Once all the cake layers are stacked, spread a thin coat of blackberry peach buttercream around the cake to fully cover the cake layers.
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 buttercream to the cake, and smooth using a bench scraper.
Divide the remaining buttercream into three bowls, and color 3 different shades of purple using gel food coloring.
Decorate as desired! Chelsey chose to add a textured look with the reserved white frosting and the ombre purple buttercream, and decorated the top of the cake with buttercream dollops and fresh blackberries.
◦ Be sure to properly measure your flour (spoon into the cup measure, then level) or use a kitchen scale.
◦ Make sure your ingredients are at room temperature to help them mix together better.
◦ Don’t overmix your batter, stir until the flour is just combined.
◦ Don’t level and torte your cake layers until they are completely cooled.
◦ Pipe a ring of buttercream around each cake layer before adding the jam and fresh berries. It will act as a frosting dam and keep all that delicious filling in place.
◦ If you have a lot of fresh or frozen blackberries on hand, make Chelsey’s homemade blackberry freezer jam! It’s absolutely delicious and makes a wonderful cake filling.
◦ Make sure your buttercream is the right consistency. This will help give your cake proper structure and make it easier to decorate.
◦ If your cake layers turn out less than perfect, read Chelsey’s cake troubleshooting guide to see where things might’ve gone awry.
◦ Make your cake layers in advance and freeze them. It breaks the process up and make 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.
◦ This blackberry cake filling can be made up to two weeks in advance if stored in an airtight container in the fridge.
◦ A frosted cake can last in the fridge for up to a week, or in the freezer for up to a month. The buttercream locks in all the moisture, keeping the cake fresh and delicious!
◦ If you cut into the cake and have leftovers, use any remaining frosting to cover the cut section to keep it moist and store in the fridge for up to a week.