Step 1: Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans or three 8-inch round cake pans. Line the bottoms with parchment paper.
Step 2: In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Set aside.
Step 3: In a large bowl, cream together the softened unsalted butter and granulated sugar with an electric mixer until light and fluffy, about 3-5 minutes.
Step 4: Add the eggs one at a time, beating well after each addition until fully incorporated. Stir in the vanilla extract and lemon zest (if using).
Step 5: Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix on low speed until just combined. Do not overmix.
Step 6: Gently fold in the fresh blackberries. If desired, add a few drops of purple food coloring and mix until the desired color is achieved.
Step 7: Divide the batter evenly among the prepared cake pans. Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean. Baking time may vary slightly if using three 8-inch pans.
Step 8: Let the cake layers cool in the pans for 10-15 minutes before inverting them onto a wire rack to cool completely. Ensure cakes are entirely cool before frosting.
Step 9: While the cakes cool, prepare the cream cheese frosting: In a large bowl, beat the softened cream cheese and softened unsalted butter with an electric mixer until smooth and creamy.
Step 10: Gradually add the sifted powdered sugar, 1 cup at a time, beating on low speed until combined, then increase to medium-high and beat until fluffy. Add the vanilla extract and beat well.
Step 11: If the frosting is too thick, add heavy cream 1 tablespoon at a time until it reaches your desired consistency.
Step 12: Once the cake layers are completely cool, assemble and frost the cake. Place one cake layer on a serving plate, spread a generous layer of frosting, then repeat with the remaining layers. Frost the top and sides of the cake. Garnish with additional fresh blackberries if desired.