Preheat the Belgian waffle maker according to the manufacturer's instructions.
In the stand mixer bowl fitted with the whisk attachment, beat the egg whites with the cream of tartar on medium-high until frothy. Add 2 tablespoons of the granulated sugar, increase speed to high, and whip until soft peaks form. Transfer whites to a clean bowl and set aside.
Switch to the paddle attachment. In the mixer bowl, beat the egg yolks with the remaining sugar until thick and pale, about 1–2 minutes. Mix in 1 teaspoon vanilla extract and 1/4 teaspoon salt.
Whisk together the flour and baking powder in a small bowl. Fold the dry mixture into the yolk batter until mostly incorporated with a few streaks remaining.
Fold one-third of the whipped egg whites into the batter to lighten it, then gently fold in the remaining egg whites until just combined; do not overmix.
Scoop about 1 heaping cup of batter (adjust for your waffle iron) into the preheated waffle maker and cook until golden brown and crisp, about 3 minutes 30 seconds or according to your machine. Repeat with remaining batter.
To make the mascarpone topping, in the stand mixer fitted with the paddle attachment beat the mascarpone, 1/2 cup sugar, and 1/4 teaspoon vanilla until smooth. Add the heavy cream and beat until thick and smooth; add 1 tablespoon more cream if the mixture is too stiff.
Pour the lukewarm espresso into a shallow bowl. Briefly dip the bottom third of each waffle into the espresso, then place waffles on plates or a cake stand for stacking.
Top each espresso-dipped waffle with a generous spoonful of the mascarpone mixture, or assemble a waffle cake by layering about 3/4 cup filling between espresso-dipped waffles.
Dust assembled waffles or waffle cake with cocoa powder and sprinkle with chocolate curls if using, then serve.