Preheat oven to 350°. Line a 9″x 13″x 2″ pan with aluminum foil.
Place butter and chocolate in a microwave-safe bowl. Microwave at 30-second intervals until melted. Stir until smooth. Set aside to cool slightly.
Whisk together flour and salt. Set aside.
In a large bowl, combine chocolate mixture and 2 & 1/2 cups sugar using an electric mixer at medium speed. Mix until well blended. Add eggs, one at a time, beating well after each addition. Mix in vanilla.
Gradually add flour mixture, beating just until combined. Stir in peanut butter chips. Spread batter in prepared pan.
Combine peanut butter and 2 tablespoons sugar in a small microwave-safe bowl. Microwave for 30 seconds. Stir until smooth.
Drop peanut butter mixture by teaspoonfuls over brownie batter. Use a knife to swirl peanut butter mixture into brownie batter. Sprinkle top with peanut butter cups.
Bake for 40-45 minutes, or until a wooden pick inserted in the center comes out still slightly sticky. Cool completely in pan on wire rack.