Start by pouring your entire box of rice cereal squares into a large mixing bowl. You'll want plenty of room to toss everything together later, so don't be afraid to use your biggest bowl for this step.
In a double boiler or microwave-safe bowl, melt the butter first, then stir in the chocolate chips and peanut butter until fully combined and glossy. Add the vanilla extract at the end and give it one more good stir.
Slowly drizzle the warm chocolate mixture over the cereal a little at a time, stirring gently with a wooden spoon. Take your time here, since rice cereal squares break easily if you toss them too roughly.
Divide the chocolate-coated cereal between two heavy-duty freezer bags, then pour in the powdered sugar. Seal each bag and shake well until every piece is evenly dusted in white.
Pour the sugared cereal onto a baking sheet, sprinkle with a pinch of salt, and toss again. Finish by mixing in the Reese's Pieces, then transfer everything into a serving bowl.