1. Prep Cakes: Preheat oven to 350°F (177°C). Grease and flour two 8-inch round cake pans and line with parchment.2. Mix Dry: In a large bowl, whisk together flour, sugar, black cocoa powder, baking soda, and salt.3. Mix Wet: In a medium bowl, whisk together buttermilk, oil, eggs, vanilla, and vinegar.4. Combine Batter: Pour wet ingredients into dry and whisk until just combined. Do not overmix.5. Bake: Divide batter between pans. Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.6. Cool Cakes: Cool in pans for 10 minutes, then invert onto a wire rack to cool completely.7. Prep Berries: Puree the blackberries in a blender. Strain through a fine-mesh sieve to remove seeds. You should have about 3/4 cup of puree. Set aside.8. Make Roux: In a small saucepan, whisk the sugar, flour, and milk. Cook over medium heat, whisking constantly, until it becomes a very thick paste.9. Cool Roux: Transfer the roux to a small bowl. Press plastic wrap directly onto the surface and let it cool COMPLETELY to room temperature.10. Whip Frosting: In a large bowl, beat the softened butter until light and fluffy (3-4 minutes). On low speed, add the cooled roux one spoonful at a time.11. Finish Frosting: Add 1/2 cup of the strained blackberry puree and the vanilla. Whip on high speed for 3-5 minutes until it resembles a light, stable whipped cream.12. Assemble: Level the cooled cakes. Place one layer on a plate, top with half the frosting. Add the second layer and frost the top and sides.