In a shallow dish, combine coffee and 1 tablespoon sugar. Dip ladyfingers in coffee mixture, one at a time, and arrange in a single layer in the bottom of a 9×13 inch baking dish. In a medium bowl, beat butter and mascarpone cheese until smooth. Beat in powdered sugar and cocoa powder. Spread half of the cheese mixture over the ladyfingers in the baking dish. Repeat layers. Cover and chill for at least 4 hours before serving.