Add the egg yolks, half of the sugar and a pinch of salt to a large mixing bowl. Using a hand mixer (see notes), beat the egg yolks and sugar until very pale and ribbony, about five minutes.
Add the mascarpone and vanilla, if using, to the egg whites and whisk together until thoroughly combined. Set aside.
Clean the beaters of the hand mixer thoroughly. In a separate large mixing bowl, beat the egg whites with a hand mixer until they reach soft peaks. With the mixer running, slowly add in the remaining sugar. Beat until they reach stiff peaks - being careful not to overbeat.
Using a spatula, transfer 1/3 of the egg whites to the egg yolk and mascarpone mixture. Gently fold together to lighten the mixture. Add the remaining egg white to the mixture and fold gently to combine, ensuring there are no remaining streaks of egg white. Set aside.
Pour the coffee into a shallow dish. Working one at a time, briefly dip each side of a ladyfinger in the coffee mixture and place in a 20cm x 20cm (8in x 8in) baking dish, lining the bottom of the dish.
Evenly spread 1/2 of the mascarpone and egg mixture over the ladyfingers. Dip and layer the remaining ladyfingers over the mascarpone mixture. Top with the remaining mascarpone and egg mixture and spread it out evenly. Cover with plastic wrap and allow the tiramisu to chill for at least 4 hours before serving.
Before serving, dust the top of the tiramisu with cocoa powder. Slice into 6 equal servings.
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Notes
You can also use a stand mixer for this recipe. In step one, fit the stand mixer with the paddle attachment and proceed as the recipe suggests. Transfer the egg yolk mixture to a large mixing bowl.Switch to the whisk attachment to whip the egg whites. Make sure the bowl is completely cleaned out before whipping. Proceed with the recipe as written.