In a heatproof bowl, whisk together the egg yolks and sugar.
Place the bowl over a pot of simmering (not boiling) water (double boiler method).
Continuously whisk for 5 minutes until the mixture is pale, thickened, and reaches a ribbon-like consistency. Remove from heat and let it cool completely.
Once the egg mixture has cooled, mix in the mascarpone until smooth and fully incorporated. Cover and refrigerate while preparing the cream.
In a large bowl, whip the heavy cream to stiff peaks.
Gently fold the mascarpone mixture into the whipped cream in 2 additions, being careful not to deflate the mixture. Refrigerate for 20 minutes.
Mix water and espresso powder and bring to a boil. Let it cool until warm
Dip the ladyfingers into the warm coffee for 2-3 seconds (don’t oversoak) and arrange a single layer in a 9x13-inch baking dish.
Spread half of the cream mixture evenly over the ladyfingers. Repeat with another layer of ladyfingers and the remaining cream.
Smooth the top, cover with plastic wrap, and refrigerate overnight.
Dust the top evenly with cocoa powder before serving