Use glass, metal or porcelain cups measuring about one cup each. (Custard cups, or something in that shape, are perfect.) Freeze the empty cups for a few hours before starting the recipe.
Put about 3 Tbs. vanilla ice cream into each cup, pressing it with a spoon along the sides of the cup to leave a deep depression. Return the cups to the freezer so the ice cream will freeze in place.
While waiting, cut sponge cake into squares about an inch on a side. Spoon a bit of the two liqueurs onto each piece of cake.
When the ice cream cups are frozen, put about 3 Tbs. of chocolate ice cream into the depressions in the vanilla ice cream. Top that with a square of liqueur-soaked sponge cake. Cover the cake with chocolate ice cream to the top of the cup. Smooth the ice cream down on top. Return to the freezer until frozen hard.
Unmold the tartufi by running warm water on the outside of the cup until the ice cream drops out onto a plate. Spoon 1/2 tsp. liqueur over each. Return to the freezer until ready to serve--or serve immediately.