To make the sponge cake
Preheat the oven to 350 degrees F. Grease and lightly flour an 8 by 11 inch sheet tray.

Heat the milk and butter over medium to high heat until the butter melts. Set aside.
Combine the flour and baking powder together.

Put the eggs, sugar, and yolks in a bowl over a hot water bath and keep whisking until the egg mixture triples in size and its color turns pale yellow.  Remove from heat and fold in the flour mixture and the milk. Pour the final mix in the greased and floured pan and bake in the preheated oven for 10 minutes.

To make the mascarpone cream
Combine the Espresso coffee, the water and the Grand Marnier and set aside.
In a large bowl add the mascarpone and rum and beat with an electrix mixer or a wire whisk, until smooth.

In a medium bowl, beat the eggs yolks, and 3 Tbsp of sugar until smooth. Set the bowl over a hot water bath and beat for 3 more minutes until light and foamy.

Remove from the heat and immediately beat this mixture into the mascarpone mixture. Set aside.
Whip the cream until the cream holds a firm shape. Fold in the vanilla.

In two small batches, fold about 1/3 of the mascarpone mixture into the whipped cream. Then fold the whipped cream into the remaining mascarpone mixture. Set aside.

Beat the egg whites and salt on medium until foamy. Increase the speed and add the remaining 3 Tbsp sugar, beat until glossy, not dry. Fold the egg whites, all at once into the mascarpone.

Tiramisu assembly
Cut the sponge cake in half crosswise. Place in a pan large enough to fit the halved sponge cake. Moisten the cake with half of the espresso mixture. Spoon half of the mascarpone mixture and spread it evenly with a spatula. Sprinkle generously with cocoa powder, and powdered sugar.

Top with the other half of the sponge cake layer and moisten with the remaining espresso mixture. Pour the remaining mascarpone mixture and spread it evenly. Finish the cake with sifted cocoa powder and icing sugar.

Refrigerate, uncovered, for at least 6 hours.