Line a 9-inch cake pan with non-stick aluminum foil, or with lightly-greased regular foil. Set aside.
In a large, greased bowl, place the cereal.
In a large, microwave-safe bowl, place the marshmallows and butter. Heat for 1 minute. Stir very well, using the residual heat to fully melt the marshmallows and butter. Stir until very smooth.
Pour the marshmallow mixture over the cereal and gently stir to fully combine.
Transfer the mixture to the cake pan and press evenly. TIP: Use an 8-inch cake pan to help flatten the treats. Let set for about 2 hours, or 1 hour in the fridge.
Prepare royal icing, divide, and tint any preferred colors.
Prepare decorating bags with couplers and tips, fill with your icing, and close tightly with rubber bands.
Decorate! The icing will fully set overnight.
Notes
For food coloring, I use Americolor Soft Gel Pastes: Super Black, Red Red, Electric Pink, Electric Green, Leaf Green, Egg Yellow, Orange, Sky Blue. (You needn't use that many colors.) You can find my royal icing recipe at thedecoratedcookie.com/royal-icing-recipe/