Cut across middle of sponge cake forming two layers, each about 1 1/2 inches high.
Add coffee and brandy or rum to a small bowl. Mix well.
Sprinkle enough of coffee/brandy or rum mixture over bottom 1/2 layer of cake to flavor it strongly. Just a nice sprinkling - Don't soak cake too much or it may collapse on serving.
Add room-temperature Mascarpone and 1 cup sugar to a mixing bowl. Beat until sugar is completely dissolved and cheese is light and spreadable.
Taste about halfway through beating. Add more sugar if needed.
Spread 1/2 of the sweet Mascarpone over cut surface of bottom layer (the one you sprinkled).
Place second layer, cut side down, atop lower layer.
Spread remaining sweet Mascarpone over top.
Sprinkle top liberally with sifted cocoa powder.
Refrigerate cake for at least 2 hours before cutting and serving.