You need a large 9×13-inch baking pan/dish with at least a 12-14 cup capacity. Set aside and have ready to go!

Whisk the espresso and Grand Marnier together in a shallow bowl. One at a time, quickly dip each side of the ladyfinger into the espresso mixture. Arrange the dipped ladyfingers in the baking pan to make one solid layer.

With a handheld or stand mixer fitted with a paddle or whisk attachment, beat the mascarpone and rum together on medium speed in a large bowl for a few seconds, just to combine. Set aside.

Prepare a double boiler for the egg yolks. Whisk the egg yolks and ¼ cup (50g) of granulated sugar together until light and foamy, about 5 minutes. Remove from the heat and immediately pour into the mascarpone mixture. Beat on medium speed until combined.

With a handheld or stand mixer fitted with a whisk attachment, beat the heavy cream and vanilla extract together on medium-high speed until medium peaks form, about 3 minutes. Fold the whipped cream into the mascarpone mixture.

With a handheld or stand mixer fitted with a clean whisk attachment, beat the egg whites and salt together on medium-high speed until foamy, about 1 minute. Increase to high speed and slowly pour in the remaining ¼ cup (50g) of sugar. Beat until stiff peaks form, about 4-5 minutes. Fold into the mascarpone cream.

Spread half of the mascarpone cream evenly over bottom layer of ladyfingers.

Dip remaining ladyfingers into remaining espresso mixture and arrange one-by-one on top of the mascarpone layer. Spread remaining mascarpone mixture evenly on top. Refrigerate uncovered for 2-3 hours.

After a couple hours in the refrigerator, sift or strain a dense layer of cocoa powder all over the top. Cover with plastic wrap or aluminum foil, then refrigerate for at least 8-9 more hours and up to 1 day.

Using a sharp knife, slice chilled tiramisu into servings. Wipe knife clean between cuts. Cover leftover tiramisu and store in the refrigerator for up to 3 days.