Preheat the oven to 350 degrees. Line an 8-inch square baking pan with foil, letting the foil over hang the edges by a few inches; grease the foil with butter or cooking spray.

In a microwave safe dish (2-cup glass measuring cup works great for this), add your chocolates, cocoa, and butter. Microwave, stirring often, for 1 to 3 minutes until the mixture is smooth. Let it cool slightly.

In a large bowl, whisk the sugar, eggs, vanilla, and salt together. Whisk in the melted chocolate mixture until combined. Stir in the flour until just incorporated. Scrape the batter into the prepared pan and smooth out the top.

Drop the peanut butter on top of the brownie batter by little spoonfuls all over the top. Use a knife to cut through the peanut butter and pull it through the brownie layer a bit. Bake the brownies until a tooth-pick inserted into the middle comes with just a few soft crumbs attached, 35 to 40 minutes.

Let the brownies cool completely in the pan; set on a wire rack. Remove the brownies from the pan by using the foil, cut into squares to serve.