2 (18 1/4 ounce) boxes devil's food cake mix
Oil, eggs and water as called for on cake mix boxes
2 cups heavy cream
3 tablespoons butter
2 (12 ounce) bags semisweet chocolate chips
1 tablespoon instant coffee powder or crystals
Prepare and bake both cake mixes in 9-inch round pans, as directed on the packages, using oil, eggs and water as directed, making four layers. (If you have only two pans, bake one mix at a time, reusing the pans after the first two layers have been removed.) Cool layers as directed on the boxes. Only after all four layers are removed from the pans and are fully cool, prepare the ganache:
Put the cream and butter in a medium saucepan. Over medium heat, stirring occasionally (but watching closely because it can boil up), bring the cream to the boil. Meanwhile, put the chocolate chips and coffee powder in the large bowl of an electric mixer. When the cream mixture has boiled, pour it into the bowl. Stir well, then beat at low speed, scraping the bowl, until the chocolate chips have completely melted and the mixture is smooth.
Pour off 1 cup of the ganache mixture into a small saucepan. Cover it and set it aside in the refrigerator.
Set aside the best-shaped, flattest-topped of the four layers. Break the other three layers into chunks and add them to the remaining ganache in the mixing bowl. Stir through with a spoon or spatula, to distribute the ganache, then beat with the electric mixer at low speed for 1 minute.
Center a 20-inch long piece of plastic wrap over each of two clean, 9-inch round cake pans. Divide the ganache-cake mixture evenly between the two plastic-lined pans. Pack it into each pan so it is firm, smooth and level. Fold the extra plastic wrap over to cover the top of each layer and refrigerate for 4 hours or more (or freeze for 2 hours).
Rewarm the set-aside ganache over low heat, stirring, just until it flows nicely. If necessary, slice a bit off the top of the set-aside plain-cake layer to make it flat and level.
Remove the two ganache-cake layers from the refrigerator. Peel the wrapping away from the top of one layer, cover it (centered) with an inverted serving plate. Invert the pan and plate together, lift away the pan and peel away the wrapping. Paint the top of that ganache-cake layer with about 1/4 cup of the re-warmed ganache, to help it adhere to the next layer.
Center the level-topped layer of plain cake on top of the first layer. Cover this layer with another cup or so of the warm ganache.
Unwrap the second layer of ganache-cake and lift it onto the first two layers using several spatulas.
Pour the remaining warm ganache over the top, spreading it while it is still warm to completely cover the top, letting it drip down the sides.
Serve or refrigerate for later, letting it warm an hour before serving.