Preheat the oven to 350℉. Pour the sugar into a mixing bowl and drizzle the melted butter overtop. Stir for 1-2 minutes to help the sugar dissolve. Stream in the vanilla extract, eggs, and egg yolk, and stir until combined.
In a microwave-safe bowl, heat the chocolate in 30-second intervals, stirring in between, until fully melted. Add the chocolate to the wet ingredients and stir.
Sift in the flour, cocoa powder, baking powder, and salt and fold together until no raw flour remains visible. Gently fold in the chocolate chips and chopped peanuts until evenly distributed. Pour the batter into a prepared baking pan and smooth into a single layer with the rubber spatula.
Using a teaspoon, drop dollops of peanut butter around the pan. Insert the tip of an offset spatula or butter knife to gently twist and swirl the peanut butter and chocolate batter together. Keep it to 1 swirl; overengineering creates a muddled look, so keep your strokes minimal and intentional.
Bake for 30 minutes, or until a toothpick inserted in the center comes out with a few wet crumbs. Allow the brownies to fully cool in the pan on a wire cooling rack, then remove from the pan and cut into squares.
