Put the chocolate chips and the boiling water in a blender; blend until smooth. Add egg yolks, cognac (you can skip this if you are making cake for kids), and butter and continue blending until smooth. The frosting will be creamy but may appear a little runny. Pour it into a bowl and cover it loosely; refrigerate for 2 hours until it is thick and creamy.

Carefully slice the poundcake into seven horizontal layers (or six, if that's all you can get out of it). Spread frosting carefully over the first layer, top with the second layer of cake, frost again, and repeat until the layers are neatly stacked and frosted. Spread remaining frosting on the top and outside of the loaf.

Chill until ready to serve, for at least an hour. Slice in rectangular pieces about 1/2 inch thick.