Line a 10×10-inch baking pan with aluminum foil that overhangs all four edges. Lightly coat foil with cooking spray.
For the crust, beat the powdered sugar, peanut butter, and butter in the bowl of an electric mixer. When combined, add the crushed pretzels. Press into the prepared pan and refrigerate until firm.
For the filling, combine the peanut butter, unsalted butter, and chips in a saucepan. Cook over low heat until melted and smooth (you may also microwave at 30 second intervals, if you’d rather). Pour in the mini marshmallows and fold together. Pour over the crust and smooth. Sprinkle with white nonpareils and confetti sprinkles, if desired. Refrigerate until firm, about 30 minutes.