For the tiramisu, first boil the espresso, chill it and as soon as it is cold, mix it with the rum. Beat the egg yolks with the sugar until creamy and let the mascarpone be stirred in by the spoonful.
Dip part of the sponge cake briefly in the coffee-rum mixture and place in a baking dish. Spread 1/3 of the cream on top, again dip biscuits into the coffee and repeat the process until the cream is used up.
Sprinkle the tiramisu with cocoa before serving.