Line 6x2-inch, 8x2-inch and 10x2-inch round cake pans with foil, leaving edges of foil hanging over sides of pans. Spray with cooking spray.
Place 1/3 cup butter in 4-quart microwavable glass bowl. Microwave uncovered on High about 1 minute or until melted. Add 7 cups marshmallows; toss to coat. Microwave about 2 minutes, stirring after each minute, until marshmallows are completely melted and mixture is well blended. Add 10 cups cereal; mix well.
Press firmly into 10-inch foil-lined cake pan. Repeat step 2 with remaining butter, marshmallows and cereal. Divide cereal mixture between 6- and 8-inch foil-lined cake pans; press firmly. Allow cake layers to cool completely, about 1 hour.
To assemble, remove cereal layer from each pan; remove foil. On cake plate, place 10-inch layer, top side up. Top with 8-inch layer, top side up. Top with 6-inch layer, top side up. Decorate with fruit snacks and additional cereal, if desired.
To serve, use a serrated knife to cut. Starting with the top layer, cut and serve the cake layer by layer.