Prepare the mascarpone cream: In a medium bowl, whisk the egg yolks with 4 tablespoons of sugar until pale and thick. Add the mascarpone and vanilla, and stir until smooth.

Whip the egg whites: In a separate bowl, beat the egg whites with the cream of tartar. When soft peaks form, slowly add the remaining 2 tablespoons of sugar and continue beating until stiff, glossy peaks hold.

Fold together: Gently fold the whipped egg whites into the mascarpone mixture in two additions, keeping as much air in the batter as possible. You should end up with a light, creamy filling.

Assemble the charlotte: Place a 10-inch metal ring on your serving plate. Spread a very thin layer of mascarpone on the bottom. Line the sides with ladyfingers, standing them upright like a crown.

Soak the ladyfingers: Mix the cooled coffee and Kahlua in a shallow dish. Quickly dip the ladyfingers — just a second or two so they don’t get soggy.

Assembly: Spread half of the mascarpone mixture over the base, then add a layer of dipped ladyfingers. Top with the remaining mascarpone cream, smoothing the surface.

Chill: Cover the cake and refrigerate for at least 6 hours, preferably overnight, to let the flavors meld and the structure firm up.

Finish and serve: Just before serving, dust generously with sifted cocoa powder. If you’d like, tie a ribbon around the outside for a classic charlotte cake finish.