01 - Preheat oven to 350°F. Grease and line three 8-inch round cake pans with parchment paper.
02 - Whisk together flour, baking powder, baking soda, and salt in a bowl; set aside.
03 - In a large bowl, cream butter and sugar until light and fluffy, about 3 to 4 minutes.
04 - Add eggs one at a time, beating thoroughly after each. Mix in lemon zest and vanilla extract.
05 - Stir in the fresh lemon juice; mixture may appear curdled, which is expected.
06 - Add dry ingredients in three additions, alternating with milk, beginning and ending with dry ingredients; mix just until combined.
07 - Distribute batter evenly into prepared pans and smooth tops. Bake 25 to 30 minutes until a toothpick inserted in the center comes out clean.
08 - Allow cakes to cool in pans for 10 minutes, then invert onto wire racks to cool completely.
09 - In a small saucepan, combine raspberries, sugar, and lemon juice. Cook over medium heat, stirring, for 5 to 7 minutes until berries break down. Stir in cornstarch slurry and cook 1 to 2 minutes more until thickened. Cool completely.
10 - Beat butter until creamy. Gradually incorporate powdered sugar, lemon juice, lemon zest, salt, and milk or cream until smooth and spreadable.
11 - Place one cake layer on a serving plate. Spread half the raspberry filling over the top, avoiding edges. Apply a thin layer of buttercream. Repeat with second layer and remaining filling. Top with final cake layer.
12 - Cover top and sides with lemon buttercream. Optionally decorate with fresh raspberries and lemon zest curls.