Line an 8 x 8-inch square tin with parchment, and spray generously with nonstick spray.

Add 1 tablespoon of butter to a medium saucepan. Melt the butter.

Stir in one 70 gram-portion of the marshmallows, and continue stirring until completely melted.

Remove from heat. Add the purple food coloring. (I used 8 drops.)

Stir in one 35 gram-portion of the puffed rice cereal, and continue stirring until well combined, with no dry cereal remaining.

Pour mixture onto prepared tin.

Press down with damp fingertips, distributing evenly across the surface of the tin. (Don’t press too firmly, and don’t worry if you see some thin spots here and there.)

Repeat the above steps, tinting the mixture with blue, green, yellow, orange, and red coloring.

Set pan aside and allow to cool at room temperature for at least one hour.

Using the parchment as handles, carefully lift the Rice Krispie treat slab from the tin. Transfer to a cutting board.

With a greased chef’s knife, slice into squares.

Serve immediately. Layer leftover treats between sheets of parchment and store in an airtight container at room temperature for up to three days.