Brew coffee and let it cool completely. Stir in optional liqueur, then pour into a shallow dish wide enough for dipping ladyfingers.
Whisk yolks, sugar, and water in a heatproof bowl over barely simmering water. Keep the bowl above the water and whisk continuously until thick and 160°F (71°C).
Remove from heat and whisk briefly, then cool about 20 minutes. Stir mascarpone in a separate bowl just until smooth, without beating it excessively.
Fold cooled yolk mixture into the mascarpone. Whip heavy cream to medium-stiff peaks in another bowl, then fold gently into the cheese mixture.
Dip half the ladyfingers into cold coffee for about 1 second per side. Arrange across the dish and spread half the cream over them.
Repeat with remaining dipped ladyfingers and cream. Smooth the surface, cover without touching the top, and refrigerate at least 6 hours, preferably overnight.
Dust with cocoa through a fine sieve shortly before serving. Cut into nine portions with a clean knife and lift with a broad spatula.
Keep unused tiramisu refrigerated. This version gently heats the egg mixture; do not replace that stage with simply whisking raw yolks at room temperature.
