In a double boiler, combine egg yolks and granulated sugar. Whisk them together over simmering water for about 5 minutes, or until fully dissolved, reaching a temperature of 185°F (85°C). The mixture should become pale and slightly thickened during this process.

Remove the bowl from the heat and transfer the egg mixture to a stand mixer. Beat on medium speed until it cools to about 75°F (24°C) and becomes light and airy. Meanwhile, melt semi-sweet chocolate gently and fold it into the cooled egg mixture until well combined.

In a separate bowl, whip mascarpone cheese and heavy cream together until soft peaks form. Be careful not to overwhip! Sift in cocoa powder and mix until just combined.

Gently fold the mascarpone mixture into the cooled egg-chocolate mixture using a spatula until smooth and velvety.

Dip ladyfingers into freshly brewed coffee for a couple of seconds, then layer them in a rectangular dish. Spread half of the mascarpone mixture over the cookies, repeat the process, and smooth the top.

Cover the assembled Chocolate Tiramisu with plastic wrap and refrigerate for at least 4 hours.

Before serving, whip remaining mascarpone cheese and cream with powdered sugar until light and fluffy. Pipe this decoration over the top, finishing with cocoa powder.