Place the egg yolks and sugar in a large metal mixing bowl, and set it over a pot of simmering water.
Cook, whisking, until the mixture is pale, thick, and doubled in volume (about 5 minutes).
Remove from the heat, and whisk in the mascarpone, melted chocolate, vanilla, and salt.
Whip the cream until it holds stiff peaks.
Fold the whipped cream into the chocolate mixture. Set the filling aside.
Whisk the hot water and cocoa together in a small bowl. One at a time, dip the ladyfingers into the cocoa mixture and place in a 9-inch by 13-inch pan. Line the ladyfingers up in two straight rows of twelve.
Spread half the filling over the ladyfingers, and repeat.
Spread the rest of the filling on top and refrigerate until ready to serve.
Optional: just before serving, dust the tiramisu with cocoa powder. Top with whipped cream and chocolate shavings.