In a small saucepan, combine 1 1/2 cups fresh blackberries with 2 tablespoons sugar and a splash of water. Cook over medium heat for 8-10 minutes until thickened. Mash gently and strain through a fine mesh sieve to remove seeds. Cool completely. Set aside 1/2 cup for the cake and 1/3 cup for the frosting.

Preheat the oven to 350°F (175°C). Grease two 8-inch round cake pans and line the bottoms with parchment paper. Lightly dust with flour.

In a medium bowl, whisk together flour, baking powder, baking soda, and salt.

In a large bowl or stand mixer, beat butter and sugar on medium speed until light and fluffy, about 3-4 minutes.

Add eggs one at a time, mixing well after each addition. Stir in vanilla extract and optional lemon zest.

In a separate bowl, whisk together buttermilk and 1/2 cup cooled blackberry puree.

With the mixer on low, add the flour mixture to the butter mixture in three additions, alternating with the buttermilk-blackberry mixture. Begin and end with dry ingredients. Mix just until combined. Add purple food coloring if desired.

Divide the batter evenly between the prepared pans. Smooth the tops.

Bake for 28-32 minutes, or until a toothpick comes out clean. Cool cakes in pans for 10 minutes, then turn out onto a wire rack to cool completely.

Beat butter and cream cheese until smooth and fluffy, about 2 minutes.

Gradually add powdered sugar, blackberry puree (reduced to 1/3 cup), vanilla, and a pinch of salt. Beat until light and creamy, about 3 more minutes. Chill if too soft.

Place one cake layer on a serving plate. Spread a generous layer of frosting on top.

Add the second layer, then frost the top and sides. Decorate with fresh blackberries, edible flowers, or mint leaves.