Heat oven to 350°F. Line bottom and sides of 13x9-inch pan with foil. In large bowl, mix all crust ingredients except marshmallows with electric mixer on low speed until crumbly. Press firmly into bottom of pan.
Bake 12 to 15 minutes or until light golden brown. Remove from oven. Immediately sprinkle with marshmallows. Return to oven; bake an additional 1 to 2 minutes or until marshmallows just begin to puff. Cool while preparing topping.
In large saucepan, place all topping ingredients except cereal and sprinkles. Heat over medium-low heat just until chips are melted and mixture is smooth, stirring constantly. Remove from heat; stir in 4 cups cereal. Immediately spoon warm topping over marshmallows; spread to cover. Sprinkle with candy sprinkles. Cool completely, at least 1 hour.
Using foil to lift, remove mixture from pan; remove foil. With deep 2 1/2-inch heart-shaped cookie cutter, cut out 18 hearts. Store loosely covered.