Mix brownie batter according to package directions. Fill paper-lined or foil-lined muffin cups two-thirds full.
Remove wrappers from peanut butter cups; set one in each muffin cup and press down until batter meets the top edge of the candy. Bake at 350° for 20-25 minutes. Cool in pan for 5 minutes before removing to a wire rack to cool completely.
Editor's Note: The Test Kitchen staff found Betty Crocker's Fudge Brownie mix bakes best for this recipe.