PREHEAT oven to 350°F. Roll out cookie dough to 1/8-inch thickness on lightly floured surface. Cut into 24 cookies, using 2- to 3-inch heart-shaped cookie cutters, rerolling dough scraps as necessary. Place, 2 inches apart, on ungreased baking sheets.
BAKE 7 to 9 minutes or until lightly browned. Cool 1 minute; remove from baking sheets to wire racks. Cool completely.
TOP bottom side of each of 12 of the cookies with 1 marshmallow. Place 4 of the cookies on microwavable plate. Microwave on HIGH 15 to 20 seconds or until marshmallows begin to puff; cover with 4 of the remaining cookies, bottom sides down, to make sandwiches. Repeat with remaining cookies and marshmallows. Decorate with icing as desired.