Combine the egg yolks, sugar and Marsala wine in a heatproof bowl.
Place the bowl on a saucepan of simmering water over low heat. Whisk the ingredients together for 5 minutes (or until eggs are light, fluffy, and pale in colour). Remove the saucepan from the heat.
Place the egg mixture into a large bowl/stand mixer. Whisk the mixture at a high speed for 5 minutes until it becomes very thick. Add the thickened cream and AdVital Powder, and mix the ingredients together at a low speed until they combine.
Line a metal, ceramic or plastic container with plastic wrap.
Cover the base of the lined container with sponge fingers that have been dipped in the coffee. Cover the biscuits with half the fortified whipped cream mixture. Add a second layer of coffee-soaked sponge fingers, and then a second layer of cream mixture.
Cover the tiramisu with plastic wrap, and refrigerate it overnight.
Dust the tiramisu generously with chocolate powder before serving.