Line a 9x9 pan with aluminum foil, and lightly spray with nonstick spray. Set it aside.
Line the bottom of the pan with 16 peanut butter cups (4 rows of 4). Slice 4 additional cups in half, and place them along the edges of the pan. This should completely cover the bottom of the pan.
Slice into quarters, or crumble the remaining 20 peanut butter cups and set them aside.
Using a 1.5 to 2-quart heavy-bottom saucepan, add the chocolate chips, the heavy cream, and the sweetened condensed milk. Heat on low, and stir until all the ingredients are completely melted and smooth.
Spread the chocolate mixture over the top of the peanut butter cups lining the pan.
Evenly sprinkle the sliced or crumbled peanut butter cups over the tops of the chocolate layer. Lightly press the peanut butter cup topping into the warm chocolate.
Allow the brownies to cool to room temperature for 30 minutes.
Cover with cling film and chill in the fridge for an additional 30 minutes.
Slice the brownies and serve. Store any leftovers in the refrigerator in an airtight container for up to 1 week.