1. Preheat oven to 350°F (175°C). Line a 9×9-inch pan with parchment paper, leaving overhang for easy lifting. Lightly grease the parchment.

2. In a large microwave-safe bowl, melt 1 cup unsalted butter. Whisk in 1 cup granulated sugar and 1/2 cup brown sugar until glossy.

3. Add 3 large eggs, one at a time, whisking well after each addition. Stir in 2 teaspoons vanilla extract.

4. Sift in 3/4 cup all-purpose flour, 3/4 cup unsweetened cocoa powder, and 1/2 teaspoon salt. Fold gently until just combined; do not overmix.

5. Spread half the brownie batter evenly in the pan. Dollop 1/2 cup creamy peanut butter in spoonfuls over the batter, swirl gently with a knife.

6. Sprinkle 1 1/2 cups mini marshmallows evenly over the peanut butter layer.

7. Top with the remaining brownie batter and smooth lightly. Dollop 1/4 cup peanut butter in spoonfuls, swirl again. Sprinkle 1/2 cup semi-sweet chocolate chips if using.

8. Bake for 32–36 minutes, or until a toothpick inserted in the center comes out with just a few moist crumbs. Tent loosely with foil after 25 minutes if browning too quickly.

9. Cool completely in the pan. Use the parchment overhang to lift brownies out. Cut into 16 squares. Enjoy!