With the oven rack in the middle position, preheat oven to 350 degrees F. Line an 8 x 8 inch square pan with enough parchment paper to create an overhang of at least two inches on two sides.

Melt seven tablespoons of the butter (reserve 1 tablespoon of the stick for the pb-powdered sugar mixture) and the chopped semisweet chocolate together in a bowl in the microwave. Microwave in 20 second intervals, stirring in between until melted. Set aside to cool.

In a small bowl, stir together 1/4 cup of the peanut butter, one tablespoon of melted butter and the powdered sugar. Set aside.

Sift together the flour, cocoa, and salt. Set aside.

In a large bowl, briskly whisk together the eggs and sugars for about a minute or until mixture becomes pale and creamy. Whisk in the vanilla, followed by the remaining 2 tablespoons of peanut butter and the reserved butter/melted chocolate mixture. Use a wooden spoon or a spatula to gently stir in the dry ingredients. Mix until just combined then spread batter into the prepared pan.

 Spoon the peanut butter-powdered sugar mixture into a plastic sandwich bag. Snip a corner with scissors and pipe the peanut butter mixture in swirls on top of the batter. Next, use a toothpick or the tip of a knife to make more swirls with the peanut butter mixture.  Do just a few swirls in each section as you make your way across the mixture. Be careful not to over-swirl or you won't get that cool marbled look on top.

Bake 23-28 minutes or until a toothpick comes out with just a few crumbs when inserted in the middle of the brownies. Do not wait until it comes out with no crumbs.

Cool in the pan for at least 2 hours at room temperature or 1 hour in the fridge.