In a saucepan, combine fresh blackberries, sugar, and a splash of water over medium heat. Cook for about 10-15 minutes, stirring occasionally, until the mixture thickens and bubbles. Strain the mixture through a fine mesh sieve into a bowl to remove the seeds, and let it cool.

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

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

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

Add large eggs one at a time, mixing well after each addition. Scrape down the sides of the bowl as needed.

In a separate bowl, whisk together buttermilk, blackberry puree, and vanilla extract until smooth.

Gradually add the dry ingredient mixture to the mixer on low speed, alternating with the wet ingredients, mixing just until combined.

Divide the batter evenly between the prepared cake pans and smooth the tops. Bake for 28-32 minutes.

Once baked, remove the cakes from the oven and allow them to cool in the pans for about 10 minutes. Then turn them out onto a wire rack to cool completely.

While the cake cools, beat the cream cheese and softened butter together until smooth. Gradually add powdered sugar and blackberry puree until fluffy and well-combined.

Once the cakes are completely cool, place one layer on a serving plate and spread blackberry frosting on top. Place the second layer over it and frost the top and sides.