Stir your espresso and 3 tablespoons of rum in a bowl, then gently dip in half of your ladyfingers one at a time.
Lay your dipped ladyfingers close together across a 9-by-13 baking dish.
Whisk your sugar and yolks in a separate glass bowl, then place it over a saucepan of simmering water to steam it.
Whisk this mixture on low heat for about 10 minutes until it thickens slightly.
Remove the mixture from heat and whisk it for a few more minutes while it cools.
Use an electric mixer to beat your mascarpone and 3 tablespoons of rum, then add in your yolk mixture.
Beat your heavy whipping cream in another bowl, then use a spatula to fold it into the mascarpone in two parts.
Spread half of this cream over the ladyfingers, then add another layer of dipped ladyfingers on top.
Spread the remaining cream on top, then top it off with a dusting of cocoa powder.
Refrigerate your tiramisu overnight before serving.
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