Preparation

1. Preheat oven to 325°. Place butter and chocolate in a heatproof bowl set over a pan of simmering water (make sure water is not touching the bottom of the bowl).  Stir chocolate mixture until melted and smooth. Remove from heat and gradually whisk in both sugars. Whisk in eggs, 1 at a time, until combined. Continue whisking vigorously for 1 full minute (this will help give a glossy top to the finished brownie). Whisk in cocoa and salt, then fold in flour to combine.  
2. Pour about one third of the brownie batter into the HexClad 4.5QT Deep Sauté Pan. Use the back of a spoon or an offset spatula to spread it evenly. Dollop about half the peanut butter in heaping teaspoons over the batter, spaced roughly an inch apart. Use a butter knife to swirl. Pour remaining brownie batter over the top and spread in an even layer. Dollop remaining peanut butter over the batter and use your butter knife to create a swirl effect.  
3. Bake on the center rack until the center is no longer jiggly and a skewer inserted into the center comes out with only a few moist crumbs, 40 to 50 minutes. Let cool at least 15 minutes, then serve scooped into bowls with vanilla ice cream.