Melt butter in a large saucepan over low heat. Add marshmallows; cook, stirring occasionally, until completely melted.
Spray a rubber spatula with nonstick cooking spray. Fold cereal into marshmallow mixture using spatula until completely coated.
Line a large baking sheet with parchment paper; lightly spray with nonstick cooking spray.
Working quickly, scoop out ¼ cup cereal mixture and shape into a heart using oiled hands or a 3-inch heart-shaped cookie cutter. If mixture cools and becomes hard to work with, warm over low heat until mixture softens up again. Place each piece on prepared baking sheet and let sit for 10 minutes.
Microwave morsels in small, uncovered, microwave-safe bowl on MEDIUM-HIGH (70%) power for 45 seconds; STIR. Morsels may retain some of their original shape. If needed, microwave for an additional 10 seconds or until of drizzling consistency.
Working quickly, drizzle melted morsels over treats using a fork. Decorate with sprinkles and colored sugars.