Preheat your oven to 350°F (175°C) and prepare 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, mixing well after each addition.
Mix in the blackberry puree until well combined.
In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.
Gradually add the dry mixture to the wet ingredients, alternating with milk, starting and ending with flour.
Divide the batter evenly between the prepared cake pans.
Bake in the preheated oven for 25-30 minutes or until a toothpick inserted comes 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 purple frosting by creaming together softened butter and powdered sugar.
Add the blackberry puree to the frosting and mix until combined and smooth.
Once the cakes have cooled, level the tops if necessary, and place one layer on a serving plate.
Spread a layer of frosting on top of the first cake layer, then place the second layer on top.
Frost the top and sides of the cake with the remaining frosting.
Garnish with fresh blackberries and refrigerate for 15 minutes before serving.
