1 (18 1/2-ounce) package chewy fudge brownie mix (recommended: Duncan Hines)
1 (12-ounce) package peanut butter chips or 24 miniature peanut butter cups
Preheat the oven to 350 degrees F. Line 24 regular muffin cups with paper cupcake liners.
Prepare the brownie mix according to package directions for cake-like brownies. Fill the cups half full with brownie batter. Place about 1 tablespoon peanut butter chips in the center of the batter, or press 1 peanut butter cup into the batter in each muffin cup until the batter meets the top edge of the peanut butter cup.
Bake for 18 to 20 minutes, until the cupcakes are set. When they can be handled safely, remove them from the muffin tins and let cool completely on wire racks. Store in an airtight container.