Preheat the oven to 350°F (175°C). Line an 8-inch square baking dish with aluminum foil, allowing the foil to extend over the edges for easy removal. Spray the foil with nonstick cooking spray.

In a large microwave-safe bowl, combine the butter and chopped semisweet chocolate. Microwave at 50% power for 1 minute, then stir. Repeat microwaving in 1-minute increments at 50% power, stirring after each until the mixture is fully melted and smooth.

Whisk in the eggs one at a time until combined. Add the sugar, vanilla extract, and salt, stirring well. Incorporate the cocoa powder and flour using a wooden spoon; the batter will be thick and require some effort to mix evenly.

In a medium bowl, mix together the peanut butter, confectioners’ sugar, vanilla extract, and milk by hand or with a mixer until smooth and well combined.

Spread half of the brownie batter evenly in the bottom of the prepared pan. Dollop the peanut butter mixture in clumps over the batter, then gently press and smooth it out into an even layer using the bottom of a measuring cup or glass.

Carefully spread the remaining brownie batter on top of the peanut butter layer, using a knife to smooth it to the edges. Sprinkle the chocolate chips and 2 tablespoons of the peanut butter chips evenly over the top.

Bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out with moist crumbs. Remove from the oven and transfer the pan to a wire rack to cool completely.

Once cooled, cut the brownies into 16 squares. Melt the remaining 4 tablespoons of peanut butter chips in a small microwave-safe bowl by heating for 30 seconds, stirring and repeating until smooth.

Transfer the melted chips to a small plastic bag, snip off a corner, and drizzle over the brownies. Refrigerate for 10 minutes to allow the drizzle to set before serving.