In a medium saucepan over low heat, melt marshmallows with butter, stirring continuously until smooth and glossy, about 4–5 minutes.

Add Fruity Pebbles to the warm marshmallow mixture and gently fold the cereal until evenly mixed for about 1-2 minutes.

Lay out a large piece of wax paper, lightly greasing it with butter.

Transfer the Fruity Pebbles mixture onto the greased wax paper, spreading to form a rectangle roughly 1-inch thick.

Carefully roll the mixture away from you using the wax paper to keep it compact until you reach the opposite edge.

Wrap the rolled sushi tightly in wax paper and chill in the refrigerator for 20-30 minutes.

Unwrap and slice the chilled sushi roll into bite-sized pieces of about 1 inch thick.