In 1-quart microwavable bowl, microwave chocolate chips, peanut butter and butter uncovered on High 1 minute and stir.
Take it out and mix, if not all is melted return for 30 seconds longer, then stir in the vanilla.
Pour mixture over cereal, stirring until evenly coated.
Pour into 2-gallon resealable food-storage plastic bag or large bowl with lid.
Add powdered sugar into the bag or bowl
Shake until well coated.
Add marshmallows, M&M's and pretzels and mix all together.
Store in airtight container and ready to eat for Christmas!
Notes
Because this is a little messy to mix when I make a bunch, I get the biggest zip lock bags to shake the sugar around or a very large closed container to shake.