Add the chocolate chips, peanut butter, butter and vanilla extract to a large microwave-safe bowl. Heat them in the microwave for 60-90 seconds, or until everything has begun to melt. Remove the bowl from the microwave and stir until smooth. Pour the Chex over the melted chocolate mixture and use a silicone spatula to gently stir until all of the Chex is completely coated in chocolate.
Line a 9x13 baking dish with parchment paper or aluminum foil. Spread the puppy chow evenly throughout the pan. Allow the bars to set in the fridge for 60-90 minutes or until the chocolate has completely set. Remove the bars from the fridge and cut them into desired bar sizes. Pour the powdered sugar into a small bowl and toss each bar in the sugar until it's completely coated. Store the puppy chow bars in the fridge.