In a microwave-safe bowl, melt the butter and marshmallows together in 30-second intervals, stirring until smooth.

Once melted, remove from microwave and stir in the Rice Krispies cereal until well coated.

For chocolate variation, stir in chocolate chips before pressing into the baking dish.

For peanut butter variation, mix in peanut butter with the melted marshmallows.

For colorful treats, fold in M&M's after mixing in the cereal.

For coconut variation, mix in coconut flakes before pressing into the baking dish.

Press the mixture firmly into a greased 9x13 inch baking dish.

Allow to cool for at least 30 minutes before cutting into squares.