Melt butter in a large saucepan over low heat.
Add marshmallows and stir until melted.
Stir in the cereal until coated.
Coat a 9 x 13 baking pan with butter or cooking spray. Press the cereal mixture into the pan using a buttered spatula.
While the treats are still warm, sprinkle Mickey-shaped Halloween sprinkles on top and gently press with your hand to ensure they will stick to the marshmallow. Let it cool completely.
Using a Mickey-shaped cookie cutter, cut shapes out of the pan of cereal treats.
