To assemble the cake, put a crêpe down on a flat plate or serving platter, then spread an even layer of cream about the thickness of the crêpe. If you make the layer too thin, there won't be enough cream in the cake. If you make it too thick, the cake won't hold its shape when you go to cut it. There should be plenty of pastry cream, so don’t worry about running out, but don’t try to use it all.