Preheat oven to 350°F. Grease and flour two 9-inch round cake pans and line bottoms with parchment paper.

Blend 1.5 cups fresh blackberries until smooth, then strain through fine-mesh sieve to remove seeds. Reserve 1 cup puree.

Whisk together flour, cocoa powder, baking soda, and salt in a medium bowl. Set aside.

Beat softened butter and granulated sugar together for 3-4 minutes until light and fluffy.

Add eggs one at a time, beating well after each addition. Scrape down bowl as needed.

Mix blackberry puree, buttermilk, and vanilla extract together in a separate bowl.

Alternate adding dry and wet ingredients to butter mixture, starting and ending with dry ingredients. Beat on low speed until just combined.

Divide batter evenly between prepared pans and bake for 30-35 minutes until a toothpick inserted in center comes out clean.

Cool cakes in pans for 15 minutes, then turn out onto wire racks to cool completely.

Beat cream cheese and butter together until smooth. Gradually add powdered sugar, beating until light and fluffy.

Place one cake layer on serving plate, spread with frosting, and top with second layer. Frost top and sides generously.

Garnish with fresh blackberries and refrigerate for at least 2 hours before serving.