Begin by greasing a 9x13-inch baking pan or silicone molds with cooking spray or butter.

In a medium saucepan, melt 1/4 cup of unsalted butter over low heat, stirring gently to avoid burning. Once melted, add 4 cups of mini marshmallows, stirring continuously for about 3-5 minutes until fully melted and smooth.

Remove the saucepan from heat and swiftly pour in 1 cup of tonic water, stirring quickly to incorporate.

If using food coloring, divide the melted mixture into separate bowls and add a few drops of neon food coloring to each bowl.

Pour in 6 cups of Rice Krispies cereal and carefully fold in the melted marshmallow mixture until evenly coated.

Spoon the mixture into the prepared pan or molds, pressing down firmly and evenly.

Allow the mixture to cool at room temperature for 1-2 hours until set, then cut into squares.