Preheat the oven to 350°F. Grease and flour two 9-inch round cake pans. In a large mixing bowl, cream the butter and sugar until light and fluffy. Add the eggs one at a time, beating well after each addition. Stir in the lemon zest and juice. In another bowl, whisk together the flour, baking powder, baking soda, and salt. Gradually add the dry ingredients to the wet mixture, alternating with milk, beginning and ending with the flour mixture. Divide the batter evenly between the prepared pans.

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, then turn them out onto a wire rack to cool completely.

Puree the blackberries in a blender or food processor. Strain the puree through a fine-mesh sieve to remove seeds. In a mixing bowl, beat the butter until creamy, then gradually add the powdered sugar, beating until smooth. Mix in the vanilla extract and blackberry puree, continuing to beat until fully incorporated.

Place one cake layer on a serving plate and spread a generous amount of blackberry buttercream on top. Place the second layer on top and apply a thin coat of buttercream to seal in crumbs. Refrigerate to set the crumb coat. Apply the final layer of buttercream and decorate with fresh blackberries and lemon slices.

Slice the cake using a sharp knife dipped in hot water for clean cuts. Serve each slice with fresh blackberries and a slice of lemon for garnish. Pair with your choice of beverage for an enhanced experience.

Store the cake in an airtight container at room temperature for up to two days. For longer storage, refrigerate and bring to room temperature before serving. To retain freshness, ensure the cake is covered and stored away from direct sunlight.