Melt the butter and chopped chocolate together in a medium saucepan over medium heat, stirring constantly for about 5 minutes, until fully melted and smooth. Alternatively, you can melt them in a microwave-safe bowl using 20-second intervals, stirring after each burst, until smooth. Once melted, remove from heat and transfer the mixture to a large mixing bowl. Let it cool slightly for about 10 minutes.

While the chocolate mixture is cooling, prepare the peanut butter filling. In a medium mixing bowl, using a handheld mixer or a stand mixer fitted with the paddle attachment, beat together the creamy peanut butter, confectioners’ sugar, vanilla extract, and milk on medium-high speed until well combined. The filling will be thick and resemble crumbly cookie dough. Set it aside.

Position your oven rack in the lower third of the oven and preheat to 350°F (177°C). Line the bottom and sides of a 9-inch square baking pan with aluminum foil, leaving an overhang on all sides for easy removal. Set the prepared pan aside.

Once the chocolate mixture has cooled, whisk in the granulated sugar and brown sugar until fully incorporated. Add the eggs one at a time, whisking well after each addition until the mixture is smooth. Stir in the vanilla extract. Gently fold in the flour, cocoa powder, and salt using a spatula, mixing just until combined.

Pour and spread half of the brownie batter evenly into the prepared baking pan. Take chunks of the peanut butter filling, flatten them with your hands, and layer the flat pieces evenly over the brownie base. Spread the remaining brownie batter on top, smoothing the surface. Sprinkle the chocolate chips and peanut butter chips evenly over the top, then gently press them into the batter with the back of a spatula.

Bake for 35–36 minutes, or until the edges begin to pull away from the sides of the pan. It’s normal for the center to crack slightly due to air trapped between the peanut butter layers. Use a toothpick to test doneness—the brownies are ready when it comes out with a few moist crumbs but no raw batter. Ovens vary, so start checking around the 30-minute mark.

Allow the brownies to cool completely in the pan set on a wire cooling rack. Once cooled, lift them out of the pan using the foil overhang and cut into squares. For clean, neat cuts, use a very sharp knife and wipe it clean with a paper towel between slices.