Beat the mascarpone with the powdered sugar, a pinch of salt, and vanilla extract in a large mixing bowl using a hand mixer or stand mixer until the sugar is incorporated into the mascarpone.
In a separate mixing bowl, beat the heavy whipping cream with the mixer until you get soft peaks.
Fold the whipped cream gently into the mascarpone-sugar mixture using a silicone scraper spatula.
Prepare a 13×9 baking dish/glass dish/casserole dish to assemble the tiramisu in. Add one tablespoon of the cream mixture and spread it on the bottom of the dish.
Add the (optional) liqueur to the cooled espresso coffee and dip the Savoiardi cookies (a couple at a time) in the coffee for about 2 seconds.
Arrange the dipped Savoiardi cookies in the dish tightly packed together.
Add half the cream mixture and even it with a scraper or spreader spatula.
Add the second layer of the dipped Savoiardi cookies.
Add the remaining cream mixture and spread it evenly. Cover the dish with plastic wrap (cling film) and refrigerate overnight (at least 12 hours).
Add cocoa powder (sifted) just before serving.