Line an 8- by 8-in. baking pan with a 12- by 17-in. sheet of foil or waxed paper.
In a large heatproof bowl set over a saucepan of hot water, use a heatproof spatula or wooden spoon to stir together chocolate, cream, Grand Marnier, and marmalade until chocolate is melted. Scrape chocolate mixture into prepared pan, smoothing top.
Chill until firm, at least 2 1/2 hours or (covered with plastic wrap) up to 1 week.
Put cocoa powder in a shallow bowl. Remove chocolate mixture from pan. With a long, sharp knife, cut chocolate mixture into 64 squares, each about 3/4 in. wide. Roll squares in cocoa powder to coat; place 1 square in each paper cup.
Gently press a candy corn into the top of each truffle. Store between sheets of waxed paper in an airtight container in the refrigerator for up to 2 weeks.
Note: Nutritional analysis is per truffle.