Line a 9x9-inch pan with parchment paper, leaving a 1-inch overhang on either side.
Add the Chex cereal to a large bowl.
Add the chocolate chips, peanut butter, and butter to a medium bowl and heat in the microwave in 30-second increments, stirring between each increment, until everything is smooth and completely melted.
Stir in the marshmallows and heat for another 45-60 seconds and stir until the marshmallows are fully melted.
Stir in the vanilla before pouring the chocolate mixture over the cereal. Stir until the cereal is evenly coated.
Transfer the cereal mixture to the prepared pan and even it out as much as you can. Let the bars chill in the refrigerator for at least an hour, until the bars are firm.
Before serving remove the bars from the pan, dust liberally with powdered sugar, and sliced into bars.