Make the Brownie Layer:

1. Preheat oven to 350˚F (177˚C). Line an 8x8 or 9x9 inch (20x20 or 23x23 cm) square baking pan with parchment paper, leaving an overhang to lift the brownies out when they're done. Set aside.
2. Combine the butter and chocolate in a bowl or liquid measuring cup. Microwave in 20-second increments, stirring after each increment, until fully melted and smooth. Set aside.
3. Using an electric mixer, whip the eggs and sugars on high speed for a full 5 minutes. The mixture will thicken and lighten in color.
4. Add the melted chocolate and vanilla to the whipped eggs. Mix on medium speed until combined, making sure to scrape down the sides and bottom of the bowl as necessary.
5. Add the flour, cocoa powder, and salt. Mix on low speed until just combined. Set aside the batter as you make the peanut butter filling.

Make the Peanut Butter Layer:

1. Add the peanut butter and butter to a bowl. Microwave in 20-second increments, stirring after each increment, until the butter is melted and the mixture thins out.
2. Sift in the powdered sugar and mix until smooth. The filling will be around the consistency of frosting.

Assemble & Bake:

1. Spread around half of the brownie batter into the prepared pan. It will be a thin layer, but do your best to make it as even as possible. A small offset spatula makes this easier.
2. Dollop the peanut butter filling over the bottom layer using a large cookie scoop or a spoon. Don't plop it all into one spot or it will be harder to spread.
3. Spread the filling evenly on top of the bottom layer of batter. Try your best not to mix the peanut butter layer and brownie layer.
4. Spread on the remaining brownie batter, again trying your best to not mix the brownie batter and peanut butter filling.
5. Bake for 20-25 minutes, or until a toothpick comes out with moist, fudgy crumbs. You do NOT want the toothpick to come out clean or your brownies will be dry and cakey- moist crumbs is key!
6. Allow the brownies to cool completely before slicing and serving.