Preheat your oven to 350°F (175°C). Grease and flour the two round cake pans to ensure the cakes come out easily after baking.
In a large mixing bowl, sift together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
In another bowl, whisk together the buttermilk, vegetable oil, eggs, and vanilla extract until well combined.
Gradually add the wet ingredients to the dry ingredients, mixing until just combined. Do not overmix. Gently fold in the blackberry puree until evenly distributed.
Divide the batter evenly between the prepared pans and smooth the tops. Bake in the preheated oven for 25 minutes, or until a toothpick inserted into the center comes out clean.
Once baked, remove the cakes from the oven and allow them to cool in the pans for about 10 minutes. Then transfer the cakes to a cooling rack to cool completely.
While the cakes are cooling, prepare the frosting. In a mixing bowl, beat the softened butter until creamy. Gradually add the powdered sugar, alternating with the blackberry puree and vanilla extract. Beat until fluffy and fully combined.
To make the blackberry reduction, combine the fresh blackberries, sugar, and lemon juice in a saucepan over medium heat. Cook until the blackberries have broken down and the mixture thickens, about 10-15 minutes. Strain if desired, and allow it to cool.
Once the cakes are completely cooled, place one layer on a serving plate. Spread a generous layer of frosting on top, then place the second layer on top and frost the top and sides of the cake.
Drizzle the blackberry reduction over the top of the frosted cake, and garnish with fresh blackberries if desired. Slice and serve.