Cook the yolks. Whisk yolks, vanilla, 3 tbsp sugar and the salt in a bowl over barely simmering water until thick, pale and 160°F (about 5 minutes). Cool 10 minutes.
Make the meringue. Whisk the egg whites with the remaining 3 tbsp sugar over simmering water until warm (160°F). Beat off the heat to stiff, glossy peaks.
Whip the cream to medium-stiff peaks.
Build the filling. Stir the mascarpone until smooth. Fold in the yolks, then the cream, then the meringue.
Layer. Dip each ladyfinger in coffee for 1 to 2 seconds per side. Line the dish, spread half the filling, then repeat.
Chill at least 6 hours, ideally overnight.
Dust with sifted cocoa just before serving.