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Adjust rack to middle position and preheat oven to 350 degrees. Place peanut butter cups in refrigerator to chill. Melt ½ cup butter (1 stick) in a small pot over medium heat, swirling occasionally. Remove pot from heat and let cool slightly. While butter melts, coat a 9-by-13-inch baking dish with cooking spray. Line dish with parchment paper, leaving a 2-inch overhang on the two longer sides. Coat parchment paper with cooking spray.