How to Make It
Make the Cake: Preheat oven to 350°F. Grease a 9- x 13-in. baking dish and line bottom with parchment paper. Whisk together flour, cocoa, baking soda, and salt in a bowl and set aside. In a large mixing bowl, whisk together the brown sugar, coffee, oil, and vinegar until sugar is dissolved. Add dry ingredients, and stir until no lumps remain. Pour into prepared baking dish, sprinkle surface evenly with chopped chocolate, and bake until a toothpick inserted into the middle comes out clean, 25-30 minutes. (Note: a few crumbs may attach to the toothpick because this is a very moist cake.)
Make the Chocolate Crumble: Preheat oven to 300°F, and line a baking sheet with parchment paper. Stir all of the ingredients together in a large bowl until they form separate sandy clumps. Scatter the clumps across the prepared baking sheet evenly. Bake for 10 minutes, remove baking sheet from oven and stir the crumbles around before returning to the oven to bake for an additional 10 minutes. Allow to cool completely.
Make the Whipped Cream: Combine the ingredients in the bowl of a stand mixer, and whip on medium until soft peaks form
For Assembly: Remove cake from baking dish by inverting the cake onto a clean surface. Clean and dry the 9- x 13-in. baking dish. Use a long serrated knife to cut through the middle of the entire cake, creating two separate 9- x 13-in. layers. Place one of the layers back into the cleaned baking dish. Spread half of the chocolate pudding evenly across the surface. Spread half of the whipped cream evenly on top of the pudding. Then sprinkle half of the Chocolate Crumble on top of the whipped cream. Repeat the process with the second layer of cake and the remaining pudding, cream, and crumble.