Prepare the Pan: Grease a 9x13-inch pan or line it with parchment paper.
Melt the Marshmallows: In a large saucepan, melt butter over low heat. Add 4 cups of marshmallows and stir until smooth.
Combine with Cereal: Remove from heat and stir in Rice Krispies cereal until fully coated.
Add Mix-Ins: Let the mixture cool slightly (3-4 minutes), then stir in the remaining marshmallows, M&Ms, and sprinkles.
Press and Cool: Gently press the mixture into the prepared pan without compacting it too firmly. Add extra M&Ms on top.
Cut and Serve: Let cool for 30 minutes before cutting into squares.
Notes
✅ Use fresh marshmallows for the best texture. ✅ Avoid pressing the mixture too firmly to keep treats soft and chewy. ✅ For thicker bars, use an 8x8-inch pan instead of a 9x13.