Cover the bottom of an 11 x 8-inch (28 x 20 cm), 10-cup (2.5 litre) rectangular dish with ladyfingers, first dipping them in the coffee mixture for 2 to 3 seconds each. Break the ladyfingers as needed to make them fit. Cover with half of the mascarpone mixture. Sprinkle half of the cocoa powder over the mascarpone mixture. Repeat with the remaining ingredients, ending with the cocoa powder. Cover with plastic wrap and refrigerate for at least 4 hours or preferably overnight.
