Melt the butter in a saucepan and add the marshmallows. Stir the mixture until the marshmallows are melted. Now add the rice crispies and stir with the mixture. Take off the fire.
Grease a baking sheet and distribute the mixture evenly about 1 cm thick on the sheet. Let the whole thing cool and then cut into small rectangles.
Warning, very sticky! The treats also taste very good with Chocolate Rice Crispies.