Break the chocolate into pieces in a small saucepan, add the butter, powdered sugar, and 2 tablespoons of water. Melt gently over a bain-marie until smooth.
Stir in the egg yolks while still over the bain-marie, ensuring a creamy consistency.
Remove from heat and carefully fold in the stiffly beaten egg whites.
Chill the mixture, reserving one spoonful of mousse for decoration.
Butter a loaf pan and line the bottom and sides with ladyfingers, domed side against the pan.
Fill the mold by alternating layers of chocolate mousse and ladyfingers dipped briefly (3-4 seconds) in the sweetened coffee.
Refrigerate overnight.
Unmold onto a serving dish, spread with the reserved mousse, and sprinkle with grated chocolate.
