Special equipment: a 13-by-9-inch baking pan; a trifle dish
For the cake: Preheat the oven to 350 degrees F. Grease and flour a 13-by-9-inch baking pan. In a large bowl, sift together the flour, sugar, cocoa, baking soda, salt and baking powder. Add the buttermilk, oil and vanilla. Stir until just combined. Add the coffee and mix well. Pour into the pan and bake until a toothpick comes out clean, about 40 minutes.
Cool the cake completely. Crumble the cake into medium chunks, cover, and then set aside.
For the pudding: In a large saucepan set over medium heat, combine the cocoa, sugar, cornstarch and salt, and mix well. Whisk in the milk. Stir constantly with a wooden spoon while the mixture comes to a low boil and thickens, 7 to 9 minutes. Reduce the heat to very low. Stir a couple tablespoons of the hot mixture into the egg yolks, then stir the egg yolks back into the hot mixture to temper. Stir constantly until the mixture thickens to the consistency of pudding, 3 to 4 minutes. Remove from the heat and stir in the vanilla. Place plastic wrap directly on the surface of the pudding to prevent a skin from forming, and then cool.
For the whipped cream: Using an electric mixer set on high speed, beat the cream, sugar and vanilla until stiff peaks form.
Place half of the cake crumbles in the bottom of a trifle dish. Top with half of the pudding, then half of the cherries, and half of the whipped cream. Repeat the layering with the remaining cake, pudding, cherries and whipped cream. Save a few cherries or cake crumbs to garnish the top layer. Cover with plastic wrap and refrigerate for at least 2 hours or up to 1 day.