If this feels like a trick to get you to make tiramisu, then you are correct. Many people don't like tiramisu, and that's because often it is bad. Too dry or too soggy, too sweet or too boozy — there's a lot that can go wrong. But if you think of it as a very good angel food cake delicately soaked with bitter espresso and layered with a barely sweetened, custardy, pillowy mascarpone, then you may be open to the possibility that you do, indeed, love tiramisu.
Technique tip: I like to use coffee from a shop since I never seem to brew my coffee strong enough. This dessert can be made 1 to 2 days ahead.
For the cream:
Using a stand mixer or an electric hand beater, in a medium bowl, whip the egg yolks and 1/4 cup sugar until very pale yellow, tripled in volume and holds a slight ribbon when the beaters are lifted. Transfer to a large bowl.
Whip the cream, a pinch of salt, the vanilla bean, if using, and the remaining 1/4 cup sugar in the mixer bowl until you've got soft peaks. Add the mascarpone and continue to whip until you've got a soft, pillowy mixture with medium peaks.
Combine the espresso and rum in a small bowl. Using a sifter or powdered sugar shaker, dust the bottom of a 2-quart baking dish with 1 tablespoon of the cocoa powder.
Place half of the cake (or a layer of ladyfingers) in the bottom of the baking dish and, using a spoon or pastry brush, douse with half the espresso mixture. Evenly spread half the mascarpone mixture onto the cake and repeat with the remaining cake, espresso and mascarpone mixtures. Dust the top layer with the remaining tablespoon of cocoa powder. Chill for at least 4 hours before serving.