Make the ganache, right before you're ready to assemble the cake. Place the chopped chocolate in a bowl. Warm the cream in a saucepan over low heat, just until it simmers around the edges. Remove from the heat and pour over the chocolate. Let stand for 3 minutes, then stir until smooth and shiny.
Cut 6 of the doughnuts in half for a total of 12 halves. Keep 3 doughnuts whole. There will be 3 layers of cake, each with 1 whole doughnut and 4 halves.
Place a whole doughnut in the center of a 10-inch cake board, using a little buttercream under the doughnut to adhere it to the cake board. Arrange 4 halves around the sides. Spoon 1/3 of the chilled custard into all the cracks and crevices. Sprinkle with some of the coconut, toffee bits, or whatever topping you’re using for texture and crunch. Pipe 1/3 of the buttercream on top of the doughnuts.
Repeat with a 2nd and 3rd layer of doughnuts, custard and buttercream.
Pour the ganache over the top, letting it drip over the sides. Top with the coconut or other toppings before the ganache sets. Refrigerate for an hour before serving.
This cake is best served the same day it’s assembled, since doughnuts become rather stale after a day or two, but it’s still good the second day, since the custard will keep the doughnuts moist.