Line a 9"x13" baking pan with aluminum foil, letting about 2 inches hang over the edge on either side. Spray with non-stick cooking spray. Set aside.

Combine the pretzels and Chex in a large bowl. Set aside.

In a large saucepan, melt the butter and peanut butter over low heat. Add marshmallows and stir constantly until they are melted and the mixture is smooth. Pour over cereal mixture; stir until combined. Add M&M's and stir. Pour mixture into prepared pan. Gently press mixture, so it's evenly distributed in pan. Let cool.

Cut into 20 squares. Store in an airtight container at room temperature for up to 3 days.