Preheat oven to 350°F. Grease and line four 4-inch cake pans or use a muffin tin for minis.
Whisk flour, sugar, baking powder, and salt in a bowl.
Beat 1/2 cup butter until creamy. Add eggs one at a time, then vanilla. Alternate adding dry mix and milk until smooth.
Divide batter into pans. Bake 15–18 minutes until a toothpick comes out clean. Cool completely.
Beat 1 cup butter for buttercream, add powdered sugar gradually, then 2 tbsp milk and 1 tsp vanilla until spreadable.
Slice cakes in half horizontally. Spread a thin layer of buttercream, add berries, and stack. Frost very lightly so sides show.
Top with a few whole berries. Chill 20 minutes before serving.