1. Prep: Preheat oven to 350°F (177°C). Grease and flour two 8-inch round cake pans.2. Mix Dry Ingredients: In a large bowl, whisk together flour, sugar, black cocoa powder, baking soda, and salt.3. Mix Wet Ingredients: In a separate medium bowl, whisk together buttermilk, vegetable oil, eggs, vanilla extract, and white vinegar.4. Combine Batter: Pour the wet ingredients into the dry ingredients and whisk until just combined. Do not overmix.5. Bake: Divide the batter (it will be thin) evenly between the prepared pans. Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.6. Cool Cakes: Let cakes cool in the pans for 10 minutes, then invert onto a wire rack to cool completely.7. Make Filling: In a small saucepan, combine blackberries, sugar, and lemon juice. Cook over medium heat until berries burst (5-8 mins).8. Thicken Filling: In a small bowl, whisk cornstarch and cold water to make a slurry. Pour into the simmering berries, whisking constantly. Cook for 1-2 minutes more until very thick. Remove from heat and let cool completely.9. Make Frosting: In a large bowl, beat the softened butter until creamy. Gradually add powdered sugar, alternating with the heavy cream and vanilla. Add salt. Beat on high speed for 3 minutes until light and fluffy.10. Assemble: Level the cooled cakes. Place one layer on a serving plate. Pipe a dam of frosting around the edge. Fill the dam with the cooled blackberry filling.11. Finish Cake: Place the second cake layer on top. Apply a thin crumb coat of frosting and chill for 20 minutes. Apply the final layer of frosting and decorate as desired.