Step 1: Make the Cake
1. Preheat oven to 350°F (175°C). Grease and line three 8-inch round cake pans.
2. In a bowl, whisk together flour, baking powder, and salt.
3. In a large bowl, beat butter and sugar until light and fluffy (about 3–4 minutes).
4. Add eggs one at a time, mixing well. Stir in vanilla.
5. Alternate adding flour mixture and milk, beginning and ending with flour. Mix just until combined.
6. Divide batter evenly among the 3 pans. Bake 25–30 minutes, or until a toothpick comes out clean.
7. Cool cakes completely before assembling.
Step 2: Make the Blackberry Filling
1. In a saucepan, combine blackberries, sugar, and lemon juice. Cook over medium heat until berries break down.
2. Add cornstarch slurry, stirring until thickened. Remove from heat and let cool completely.
Step 3: Make the Blackberry Buttercream
1. Beat butter until creamy.
2. Gradually add powdered sugar.
3. Mix in blackberry puree, vanilla, and a pinch of salt. Adjust consistency with cream if needed.
Step 4: Assemble
1. Place one cake layer on a stand, spread with blackberry filling.
2. Add second layer, repeat filling. Top with third cake layer.
3. Frost the entire cake with blackberry buttercream.
Step 5: Garnish
