1. Prep: Preheat oven to 350°F (177°C). Grease and flour TWO 8-inch round pans (for layers) and ONE 9×13 pan (for crumbs).2. Make Cake Batter: Whisk dry ingredients (flour, sugar, cocoa, soda, powder, salt). In a separate bowl, whisk wet (buttermilk, oil, eggs, vanilla). Combine wet and dry. Stir in hot water (batter will be thin).3. Bake: Pour 1/3 of batter into the 9×13 pan (this is for crumbs) and bake 15-20 min. Divide remaining 2/3 of batter between the two 8-inch pans and bake 30-35 min. Cool all cakes.4. Make Pudding: Puree blackberries and strain to remove seeds. In a saucepan, whisk egg yolks, sugar, and cornstarch. Whisk in blackberry puree and milk.5. Cook Pudding: Cook over medium heat, whisking constantly, until boiling and very thick. Remove from heat.6. Finish Pudding: Stir in the chopped dark chocolate, butter, and vanilla until smooth. Press plastic wrap to the surface and chill completely (at least 2 hours).7. Make Frosting: Beat butter until fluffy. Sift in sugar and cocoa, alternating with cream and vanilla. Beat until light.8. Make Crumbs: Crumble the 9×13 cake layer onto a baking sheet.9. Assemble: Place one 8-inch cake layer on a plate. Pipe a dam of frosting around the edge. Fill with half the cold pudding.10. Layer: Add the second 8-inch layer. Pipe another dam and fill with remaining pudding.11. Frost: Cover the top and sides of the cake with the remaining chocolate frosting.12. Decorate: Press the cake crumbs all over the sides and top of the cake until fully coated. Chill until ready to serve.