Line an 8-inch or 9-inch square baking pan with parchment paper and coat with nonstick spray.

In a large bowl, cream 5 TBSP of butter, brown sugar, and powdered sugar until smooth. Stir in peanut butter. Mix well and press into prepared pan.

In a small microwave-safe bowl, heat chocolate chips and remaining butter for 1 minute. Stir carefully and continue to microwave in 30-second increments until smooth, stirring after each increment. Pour over peanut butter filling and spread evenly.

Chill until completely firm, at least 3 hours or overnight. Let stand for 10 minutes at room temperature before serving. Store leftovers in the refrigerator.