Press about 3/4 cup cooled fudgy brownies into six portions of about 2 tablespoons each. Shape each portion into a disc about 2 inches wide and 1/3 to 1/2 inch thick. Set aside.

Beat 6 tablespoons unsalted butter, 6 tablespoons granulated sugar, and 1/2 cup packed brown sugar until light and fluffy. Add 1/2 cup peanut butter, 1 large egg, and 1 1/2 teaspoons vanilla extract; mix until smooth.

Add 1 1/4 cups all-purpose flour, 1/2 teaspoon baking soda, and 1/4 teaspoon salt. Mix just until combined. Fold in 1 cup milk chocolate chips. Divide the dough into 12 equal portions, about 1/4 cup each.

Flatten one dough portion into a thick base about 2 1/2 to 3 inches wide. Place one 2-inch brownie disc in the center. Flatten a second dough portion over the brownie and pinch all edges shut. Repeat with the remaining 10 dough portions and five brownie discs to make six cookies.

Place all six cookies on parchment-lined baking sheets with room to spread. Refrigerate for 20 minutes while heating the oven to 350°F.

Bake the six cookies at 350°F for 13 to 15 minutes, until edges are set and tops are puffed and matte. Bake one sheet at a time if needed. Cool on the baking sheets for 10 minutes, then transfer to a wire rack. Finish with extra milk chocolate chips, chocolate drizzle, or flaky sea salt, if desired.