Line 2 9-by-13-inch baking pans with foil, leaving 2-inch overhangs on 2 sides of each pan. Mist with cooking spray. In a large pan, melt butter over low heat. Add marshmallows; stir until melted. Remove from heat, and stir in Rice Krispies. Working quickly, mist your hands with cooking spray, divide mixture in half and press into baking dishes. Let cool completely.
Let ice cream soften slightly. Working quickly, spoon ice cream on top of Rice Krispies layer in 1 baking pan, spreading evenly to edges. Remove Rice Krispies layer from other baking pan, using foil overhangs as handles. Peel off foil and place layer on top of ice cream, gently pressing down. Return to freezer until hardened, about 1 hour.
Line a baking sheet with parchment; set in freezer. Using foil handles, lift ice cream treat from pan, peel off foil and place on a cutting board. Cut into 18 sandwiches; place on baking sheet in freezer. Remove a sandwich from freezer, press edges of sandwich into sprinkles, then return to freezer. Repeat with remaining sandwiches. Freeze until ice cream hardens, at least 30 minutes. (For longer storage, wrap sandwiches in plastic wrap after they are firm.)