Arrange a single layer of ladyfingers on bottom of an ungreased 11 x 7 cake pan, one with deep sides.
Drizzle half of the espresso/rum mix across the ladyfingers.
Using spatula, spread half of the custard evenly.
Now spread half of the whipped cream on top of the custard.
Repeat this layering process, starting again with the ladyfingers and finish with a generous dusting with cocoa powder. Refrigerate overnight.