Melt the butter and chopped chocolate in a medium saucepan on medium heat, stirring constantly, about 5 minutes. Or melt in a medium microwave safe bowl in 20 second increments, stirring after each, in the microwave. Remove from the heat, pour into a large mixing bowl, and allow to slightly cool for 10 minutes.

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

Adjust the oven rack to the lower third position and preheat oven 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. Set aside.

Whisk the granulated and brown sugars into the cooled chocolate/butter mixture. Add the eggs, one at a time, whisking until smooth after each addition. Whisk in the vanilla. Gently fold in the flour, cocoa powder, and salt.

Pour/spread half of the brownie batter into the prepared baking pan. Take large chunks of the peanut butter filling and flatten with your hands. Place flat pieces in an even layer on top of the brownie layer, as shown in the photo above. Pour/spread the remaining brownie batter on top. Top with the chocolate chips and peanut butter chips. Using the back of a spatula, gently press the chips into the brownie batter.

Bake for 35-36 minutes or until the brownies begin to pull away from the edges of the pan. (The brownies may slightly crack in the center—that’s because air in between the peanut butter layers is trying to escape. That’s normal!) A toothpick inserted in the center should come out with only a few moist crumbs when the brownies are done. All ovens are different, so keep an eye on them after 30 minutes and use the toothpick test to see when yours are finished.

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