HEAT oven to 350ºF.
PREPARE cake batter as directed on package. Stir in dry gelatin mix until blended; spoon into 24 paper-lined muffin cups, filling each cup half full.
PLACE small marble or 1/2-inch ball of foil between each liner and side of muffin cup, pushing liner into batter to form a heart shape.
BAKE 20 to 22 min. or until toothpick inserted in centers comes out clean. Cool completely; remove marbles or foil balls.
FROST cupcakes with COOL WHIP. Decorate with sugar.