In a saucepan, combine fresh blackberries, a bit of sugar, and a splash of water. Cook over medium heat for about 10–12 minutes until thickened. Strain mixture through a fine sieve to remove seeds and cool the blackberry puree.

Preheat your oven to 350°F (175°C) and prepare two 9-inch round cake pans by greasing them and lining with parchment paper.

In a medium mixing bowl, whisk together all-purpose flour, baking powder, baking soda, and salt until well combined. Set aside.

In a large mixing bowl, beat softened unsalted butter and granulated sugar together until light and fluffy, about 3–5 minutes.

Add the eggs one at a time, mixing until fully incorporated. Follow with the addition of vanilla extract and optional lemon zest, combining until smooth.

Gradually add the dry ingredients alternately with the buttermilk and blackberry puree, being careful not to overmix.

Pour the batter evenly into the prepared pans and bake for 28-32 minutes, or until a toothpick comes out clean.

Let the cakes cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.

In a large bowl, beat together softened cream cheese and butter until creamy. Slowly add powdered sugar and mix until fully incorporated, then add cooled blackberry puree.

Once the cakes are cool, frost the top of one layer with blackberry frosting and add fresh blackberries between layers if desired. Place the second layer on top and frost the top and sides of the cake.