line an 8 inch square pan with parchment and lightly grease.
Make Brownie Batter
whisk melted butter with granulated sugar and brown sugar until glossy
whisk in eggs and vanilla until smooth
fold in cocoa powder then flour and salt just until combined
fold in chocolate chips if using
spread batter evenly in pan.
Make Swirl and Bake
stir peanut butter with melted butter powdered sugar vanilla and enough milk to make a thick pourable mixture
dollop peanut butter mixture over brownie batter and gently swirl with a knife
bake until edges are set and a toothpick inserted near the center comes out with moist fudgy crumbs 25 to 32 minutes
cool completely before slicing for clean squares.
Notes
1. Use natural peanut butter only if well stirred and thick; for extra chew slightly underbake and cool fully; swap chocolate chips for peanut butter chips or chopped peanuts; double in a 9 by 13 inch pan and bake 5 to 10 minutes longer