Whisk together the melted butter and both sugars. Whisk in the eggs one at a time, then add the flour, cocoa powder and salt and fold together until combined.
Pour the batter into a 9 inch square pan lined with parchment paper. Dollop the peanut butter on top then swirl it into the batter with a knife or toothpick.
Bake the brownies at 350 degrees Fahrenheit for 40-45 minutes. Let cool completely.
Fill a sauce pan with about two inches of water and bring it to a gentle simmer. In a heat proof bowl, place the egg whites, powdered sugar and cream of tartar. Place the bowl on the sauce pan over the simmering water.
Continuously whisk for about 4-5 minutes until the mixture is thin and frothy on top. Remove from heat and add the vanilla extract. Beat on high speed for 5-6 minutes until stiff peaks form.
Once the brownies are completely cool, top with the fluff, toast with a baking torch if you have one, slice and enjoy!