Measure out the Rice Chex Cereal into a large bowl, and set aside.

In a large glass measuring cup or a microwave-safe bowl, combine the peanut butter, chocolate chips, and butter. Heat in 30-second bursts in the microwave until mostly melted, then stir until smooth.

Pour the peanut butter-chocolate mixture over the Chex cereal, and stir using a rubber spatula until coated.

Sprinkle in the powdered sugar, and toss gently until coated.

Let the chocolate mixture cool for 30 minutes, then top with M&Ms and sprinkles.

Stir them together and then serve or store until later.