Whip the egg whites. Place 6 egg whites in the bowl of a stand mixer fitted with a whisk attachment, then whisk until they form very stiff peaks. Transfer to a large bowl, then wipe out the stand mixer bowl.
Make the whipped cream. Pour 2 cups (480g) heavy cream into the stand mixer bowl, then whisk until soft peaks form. Add 2 Tbsp rum (optional), 2 tsp vanilla extract, and 1 tsp instant espresso powder. If you don’t have espresso powder, add 2 Tbsp brewed coffee. Transfer to a second large bowl.
Make the zabaglione. First, prepare the double boiler. Fill a saucepan halfway with water, then bring the water to a simmer over medium heat. Place a heat-proof bowl on top (the bottom of the bowl should not be touching the water), then add 6 large egg yolks, ¾ cup (150g) sugar, ¼ cup marsala wine, and a pinch of Kosher salt. Whisk until the mixture is thick and creamy and has turned a pale yellow color, about 5 minutes. Adjust the heat as needed to maintain a gentle simmer. Remove from the heat, transfer to the stand mixer bowl, then add 1 lb (453g) room temperature mascarpone cheese. Whisk until very smooth and creamy, about 2 minutes.
Fold in the cream. Add the whipped cream to the mascarpone mixture, then fold in until well blended.
Fold in the egg whites. Add about 2 cups egg whites to the cream mixture, then gently fold until well blended. Add the remaining egg whites, then carefully fold in.
Transfer the brewed coffee to a large bowl, if you haven’t already.
Assemble the tiramisu. One at a time, dip each ladyfinger into the brewed coffee, then arrange in an even layer along the bottom of a 9x13 baking dish. There should be two straight rows of 12-13 ladyfingers. Use a spatula to spread half of the mascarpone mixture on top of the ladyfingers. Add a second layer of ladyfingers on top then spread remaining mascarpone mixture into an even layer. Dust the top of the tiramisu with cocoa powder, then refrigerate for 8 hours or overnight. Garnish with chocolate curls, if wanted. Serve immediately!