Preheat the oven to 350°F (177°C). Line an 8×8-inch baking pan with foil, leaving some overhang for easy lifting, and spray the foil with nonstick cooking spray.

In a large bowl, combine the melted ½ cup (113 g) unsalted butter, ⅓ cup (67 g) granulated sugar, and ½ cup (100 g) packed light brown sugar. Stir with a wooden spoon until smooth.

Add 1 large egg and 1 teaspoon vanilla extract to the butter-sugar mixture and stir until fully incorporated.

Add ¼ cup (17 g) unsweetened cocoa powder, ½ teaspoon baking soda, and ½ teaspoon salt. Stir until evenly combined.

Gradually add 1 ⅓ (155 g) cups all-purpose flour to the bowl and mix just until the dough comes together and is smooth. Scrape down the sides of the bowl as needed. Do not overmix.

Stir in 1 cup (170 g) chocolate chips until distributed through the dough.

Remove about half of the dough and set it aside. Press the remaining half evenly into the bottom of the prepared pan to form a thin cookie layer. If the dough sticks to your hands, spray hands lightly with nonstick cooking spray or use a spatula to press it down.

In a medium microwave-safe bowl, heat ¾ cup (200 g) peanut butter for about 45 seconds, or until it becomes smooth and slightly runny. Immediately whisk in 1 cup (113 g) powdered sugar, 1 teaspoon vanilla, and 2 tablespoons milk until the mixture is thick and fully combined.

Spread the peanut butter mixture evenly over the pressed cookie layer, leaving a small (about 1/4-inch) border around the edges.

With clean hands or a spatula, break the reserved dough into pieces and press it gently over the peanut butter layer to cover it. Seal the edges where the top dough meets the pan so the filling is enclosed. The top layer will be thin.

Bake in the preheated oven for 28–33 minutes, or until the top no longer looks glossy and a toothpick inserted into the cookie portion (not necessarily the center of the filling) comes out with a few moist crumbs but not raw batter.

Cool the bars completely in the pan on a wire rack (use the foil overhang to lift them out if desired) before slicing into squares. Store in an airtight container for up to 3 days or freeze for up to 2 months.