Place the 9 cups of Chex cereal into a large mixing bowl. Add the 15 Oreo cookies, crushed into small chunks or crumbs. Gently toss to distribute evenly.
In a microwave-safe bowl, combine ½ cup butter, ½ cup peanut butter, and 1 cup chocolate chips. Microwave in 30-second increments, stirring until smooth and melted. Stir in 1 teaspoon vanilla extract.
Pour the melted chocolate and peanut butter mixture over the cereal and Oreo mixture. Gently fold with a spatula or wooden spoon to coat all pieces evenly.
Transfer the coated cereal mix into a large resealable plastic bag or bowl with lid. Add 1 cup powdered sugar, seal, and shake vigorously until well coated.
Spread the muddy buddies on a baking sheet lined with parchment paper. Allow to cool and set for about 30 minutes. Store in airtight container or serve immediately.
