Preheat the oven to 350 °F and line a 9 x 13 baking dish with foil or use a nonstick one.
In a large pot melt the chocolate and butter over low heat. When melted and smooth remove from the heat and whisk to cool for 5 minutes.
Whisk in the sugars and then the eggs, one at a time. Finally add in the vanilla, salt, and flour. Pour into the baking dish.
Place the peanut butter into a baggie, snip off a small bit from the corner and then squeeze out four horizontal lines of peanut butter over top. Drag the edge of a knife across the peanut butter to create the swirls.