For the Lime Cake
Preheat the oven to 350°F. Butter and flour two round 9-inch cake pans. Line the bottoms of the pan with parchment paper.
Crack the eggs into a medium bowl. Add ¼ cup of the buttermilk and whisk the eggs and buttermilk together.
Cut the 2 sticks of butter into 12 pieces.
Place the sugar in the bowl of a stand mixer. Zest the limes directly onto the sugar until you have about 2 tablespoons. With your fingers rub the zest into the sugar, until the sugar starts to clump together a little and the sugar turns a light green.
Add flour, baking powder, baking soda, and kosher salt to the mixing bowl and mix on low speed for 30 seconds to combine the dry ingredients.
Add the butter and ¾ cup of the buttermilk mixture and mix on low speed until the ingredients are combined. Increase the mixer speed to medium and beat the ingredients for 90 seconds. The batter should appear creamy. Stop the mixer and scrape down the sides and bottom of the bowl.
Add the egg mixture in three batches, beating the batter on medium speed each time until the egg mixture is incorporated. Scrape the sides and bottom of the bowl after the second addition. After all the egg mixture is added, add the lime juice and mix on medium speed just until incorporated.
Divide the batter evenly between the two prepared cake pans. Use an offset spatula to smooth the batter.
Bake the layers for 30-35 minutes until the center of the cake layers register 202°F with an instant read thermometer or if a toothpick is inserted in the center and it comes out clean with no batter on it. The cake will also begin to pull away from the sides of the pan.
Remove the cake pans from the oven and place them each on a wire rack to cool for 10 minutes. Cover each wire rack with a piece of parchment paper slightly larger than the cake pan. Invert each cake layer onto the wire rack and allow the cakes to completely cool to room temperature before adding the blackberry buttercream.
For the Blackberry Sauce
Place a medium sized bowl with a fine-mesh strainer next to the stove.
Place the fresh blackberries, sugar, and lime juice in a medium-sized saucepan and bring it to a boil. Quickly reduce the heat to medium and cook the blackberry mixture until the blackberries are soft enough to smash with the back of a large spoon or a potato masher.
Transfer the berries to the fine-mesh strainer held over the medium bowl and use the back of a large spoon to press the berry mixture through the strainer to remove the seeds. Get as much juice and pulp out of the blackberries as possible. You should have about ¾ cup of blackberry sauce.
Return the blackberry sauce to the saucepan and cook on medium heat, stirring constantly, until it is reduced by half. You should have 6-7 tablespoons of reduced sauce, when it’s finished.
For the Blackberry Buttercream
Half fill a 3-quart or 4-quart saucepan with water and bring it to a simmer over high heat. Once it starts to simmer, reduce the heat to medium.
Place the egg whites and sugar in the bowl of a stand mixer and set it over the pan of simmering water. Do not allow the bottom of the pan to touch the water.
Whisk the mixture by hand until it reaches 165°F then immediately put the bowl on the mixer stand, attach the whisk attachment and whip at medium-high speed until the meringue mixture is cool (about 75°F-80°F).
With the mixer on medium speed add the butter to the meringue one tablespoon at a time until it is completely incorporated. About half through the buttercream will begin to look curdled. Don’t worry it’s supposed to look like this. Continue to add the butter and eventually the buttercream will begin to look creamy and smooth.
Scrape down the sides and bottom of the bowl. Add one tablespoon of the blackberry sauce to the buttercream and mix it in on medium speed. The buttercream will have a light lavender hue. If you want the buttercream to have a darker color with a more intense blackberry flavor continue to add the sauce until the desired color and flavor is achieved.
Assemble the Blackberry Lime Cake
Reserve 1 cup of the blackberry buttercream in a smaller bowl for decorations.
Place one cake layer on a cake board or plate. Use 1 ½ cups of buttercream for the center and evenly spread across the layer to the edges. Place the second cake layer on top. Crumb coat the cake by spreading a thin layer of buttercream over the entire layer of the cake to lock in the crumbs. Place in refrigerator to firm up before applying the remaining blackberry buttercream frosting. This technique will prevent cake crumbs from getting mixed into the final buttercream.
Use the remaining buttercream to frost the top and sides of the cake. Put reserved 1 cup buttercream in a piping bag and pipe a decorative border of your choice on the top of the cake. Decorate the top of the cake with fresh blackberries and fresh edible flowers.
Storing the Cake:
The cake can remain at room temperature for the remainder of the day. If serving the next day, store the cake in the refrigerator and then remove it from the refrigerator 90 minutes before serving so that it can come to room temperature.
- Use a scale to weigh the flour for the most accurate results. Too much flour will result in a dry cake and too little flour will result in a sunken cake.
- The eggs, butter, and buttermilk should all be at room temperature before combining them with other ingredients. Room temperature for these ingredients is typically between 65°F – 70°F.
- Line the cake pans with parchment paper to ensure a clean release from the pan.
- The butter for the buttercream should be at 68-70°F before adding it to the meringue. If it is too cold, it won’t incorporate and if it is too warm, the buttercream won’t set up and it might be soupy.