First, set aside a large 9x13 baking dish with at least 12 cup capacity. Start with the mascarpone cream. In a medium bowl with a handheld mixer, beat the mascarpone and rum together on medium speed. Once the mixture is smooth, set aside. In another medium bowl with a handheld mixer, beat the heavy whipping cream and vanilla extract together on medium high speed for about 4 minutes or until medium peaks form. Be careful to avoid whipping the mixture into butter. Once the medium peaks have formed, fold into the mascarpone mixture.
Next, beat the egg whites. In a separate bowl with a handheld mixer, beat the egg whites and salt together until foamy. Once foamy, increase to a higher speed and add in 1/2 cup of sugar. Beat until stiff peaks form, then fold into the mascarpone/whipped cream mixture.
Next, prepare a doubt boiler for the egg yolks. Place a heat-proof bowl over a pot of simmer water on medium to medium-low heat, ensuring that the water doesn’t touch the bottom of the bowl. Use a whisk to beat the egg yolks and granulated sugar together until foamy, about 5 minutes. Once foamy, remove from heat and temper into the mascarpone mixture. Once tempered, beat with a handheld mixture on medium speed until thoroughly combined.
Now it is time to assemble the tiramisu. In a shallow bowl, whisk together the marsala wine and espresso. Quickly dip the 1st layer of ladyfingers into the mixture, about 2 seconds per side. Place in a single layer in the baking dish. You may need to cut a few ladyfingers to fill in any empty spaces. Once this first layer is complete, cover with a layer of the mascarpone cream, spreading it evenly with an offset spatula. Dip the remaining ladyfingers into the espresso/Marsala wine mixture and arrange on top of the mascarpone layer, gently pressing down each ladyfinger. Spread the remaining mascarpone mixture on top.
Sift a dense layer of cocoa powder over the top. Wipe the rim of the dish with a damp paper towel, cover and refrigerate overnight or for about 9 hours. Once chilled, slice the tiramisu into servings with a sharp knife, cleaning the knife between the cuts.