1. Make ahead: For the pudding: whish sugar, cornstarch, salt, half and half, and milk in a large saucepan and set over medium heat. Add choclate and whisk constantly until chocolate melts and mixture begins to bubble, about 2-4 minutes. Stir in vanilla and transfer to a large bowl. Place plastic wrap directly on surface of pudding and refrigerate til cold, at least 4 hours or up to one day.

2. For cake layers: Put oven rack in middle position and heat oven to 325. Butter and flour two 8 inch cake pans. Whisk flour, baking pwder, baking soda and salt in a bowl.

3. Melt butter in a large saucepan over medium heat. Stir in cocoa and cook about 1 minute, til fragrant. Take off heat and whisk in coffee, buttermilk, and sugars until dissolved. Whisk in eggs and vanilla, and then whisk in flour mixture.

4. Divide batter between pans and bake til toothpick come sout clean, about 30-35 minutes. Cool in pans 15 minutes, then invert onto wire racks. Cool completely (at least 1 hour, likely more.)

5. To assemble: Cut each cake in half horizontally. Crumble one cake layer into medium crumbs and set aside. Place one cake layer on a cardboard round or cake plate. Spread 1 cup of pudding over cake layer and top with antother layer. Repeat with another cup pudding and last cake layer. Spread remaining pudding evenly over top and sides of cake. Sprinkle cake crumbs evenly over top and sides of cake, pressing lightly so they stick.

Cake can be refrigerated for up to 2 days.