Grease two 9-inch round cake pans and line the bottoms with parchment. Heat the oven to 350°F (175°C).

Beat the egg yolks with about half of the sugar until pale and thick. In a clean bowl, beat the egg whites with the remaining sugar until medium-stiff peaks form, then fold the yolk mixture and vanilla into the whites.

Sift the flour, baking powder, and salt over the egg mixture. Fold gently just until combined, taking care not to deflate the batter.

Divide the batter between the prepared pans and bake for 25-30 minutes. The tops should spring back lightly, and a toothpick inserted in the center should come out clean.

Cool the cakes completely, about 1 hour. Stir the sugar into hot espresso until dissolved, add the optional liqueur or rum, and cool the coffee syrup completely.

Beat the chilled mascarpone briefly until smooth. Separately whip the cold heavy cream, powdered sugar, and vanilla to medium-stiff peaks, then fold the whipped cream into the mascarpone in two or three additions.

Level the cakes if needed and place one layer on a serving plate. Brush it gradually with coffee syrup, spread with about one-third of the mascarpone cream, add the second layer, and repeat the syrup application.

Cover the top and sides with the remaining mascarpone cream. Dust with cocoa powder and add optional chocolate curls, then refrigerate for at least 4 hours or overnight before slicing.