In a small saucepan over medium heat, combine the blackberries and sugar. Cook, stirring occasionally, until the berries break down and the mixture thickens. Remove from heat and let cool completely.

Preheat your oven to 350°F. In a large mixing bowl, cream butter and sugar until light and fluffy. Add eggs one at a time, beating well after each addition. Stir in vanilla.

In a separate bowl, whisk together flour, baking powder, baking soda, and salt. Gradually add to the creamed mixture alternately with milk, starting and ending with the flour mixture. Mix just until combined.

Divide the batter evenly between the prepared cake pans. Bake for 25-30 minutes or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes, then turn out onto wire racks to cool completely.

In a large bowl, beat the butter until creamy. Gradually add powdered sugar, beating until smooth. Stir in vanilla and enough milk to achieve a spreadable consistency.

Place one cake layer on a serving plate. Spread with half of the blackberry filling. Top with the second cake layer. Spread a thin layer of frosting over the entire cake to seal in crumbs, then frost with remaining frosting.

Decorate as desired with fresh blackberries or other fresh berries for added visual appeal. Slice and serve the cake at room temperature for the best flavor.

Store any leftover cake in an airtight container in the refrigerator for up to 5 days. Allow slices to come to room temperature before serving for the best texture.