Place the 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 the flour and salt. Set aside.

In a large bowl, combine the chocolate mixture and 2 & 1/2 cups sugar using an electric mixer at medium speed. Mix until well blended. Add the eggs, one at a time, beating well after each addition. Mix in the vanilla.

Gradually add the flour mixture, beating just until combined or a few streaks of flour remain. Stir in the peanut butter chips. Spread the batter evenly in the prepared pan.

Combine the peanut butter and 2 tablespoons sugar in a small microwave-safe bowl. Microwave for 30 seconds. Stir until smooth.

Drop the peanut butter mixture by teaspoonfuls over the brownie batter. Use a thin knife to swirl the peanut butter mixture into the brownie batter. Sprinkle the 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 the pan on a wire rack before cutting into bars.