Preheat the oven to 325°F (160°C). Butter a 9 x 9-inch (23 x 23 cm) square brownie or cake pan.

Break up the chocolate into pieces and place in a small saucepan or heatproof bowl. Slowly melt the chocolate in the pan over very low heat or in the bowl in the microwave or over a pot with an inch of gently simmering water; remove from the heat when the chocolate is almost but not quite completely melted and stir away the heat until all the chocolate is melted. Set aside to cool to at least tepid.

Stir the flour, baking powder, and salt together in a small bowl.

Put the softened butter with the sugar in a large mixing bowl and beat until well-blended—start with your electric hand mixer on low or medium-low speed to avoid bits of butter flying, increasing mixer speed once blended until creamy.

Beat in the eggs one at a time, beating in the vanilla with the first egg and the milk with the last, beating well after each addition. Add the flour and beat on low just until blended; scrape down the sides of the bowl and beat again until creamy.

Transfer ⅓ of the batter to a smaller bowl. Beat the peanut butter into this smaller third portion of batter. Beat the melted chocolate into the larger ⅔ portion of batter.

Spoon the two batters into the prepared brownie pan, then, using the long blade of a knife, swirl the two batters together to marble, evening out the top and spreading evenly to the edges and into the corners.

Sprinkle the coarsely chopped peanuts across the top of the batter.

Bake in the preheated oven for 35-45 minutes, depending both on your oven and how gooey or cake-like you like your brownies. The top should no longer be shiny, the edges just barely beginning to pull away from the pan, and the center set.