Preheat your oven to 350°F (175°C). Grease and flour three 8-inch round cake pans.

In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.

Add the eggs, one at a time, beating well after each addition. Mix in the vanilla extract.

In a separate bowl, combine the flour, baking powder, and a pinch of salt. Gradually add this dry mixture to the wet ingredients, alternating with the buttermilk.

Gently fold in the blackberry puree, taking care not to overmix the batter.

Divide the batter evenly among the prepared cake pans and smooth the tops.

Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cakes to cool in the pans for 10 minutes before transferring to a wire rack to cool completely.

While the cakes cool, prepare the frosting by beating the softened butter until creamy, then gradually add the powdered sugar.

Add the blackberry puree and milk (as needed) to achieve the desired consistency. Beat until fluffy and well combined.

Once the cakes are cool, place one layer on a serving plate, spread a layer of frosting, and repeat with the remaining layers.

Cover the top and sides of the cake with the remaining frosting and finish with fresh blackberries.