1-First, preheat the oven to 350°F and line a 9 x 9-inch square baking pan with parchment paper on all sides to prevent sticking.

2-Then, melt the 1/2 cup unsalted butter in a microwave-safe bowl, stir in the 1 1/2 cups granulated sugar, and microwave again for 30 seconds, stir, and microwave once more for 60 seconds until slightly bubbly but not burnt.

3-Next, remove it from the microwave and stir in the 6 ounces chopped bittersweet chocolate until melted, letting it cool slightly before adding the 2 large eggs and 2 teaspoons vanilla extract, whisking until smooth and frothy.

4-After whisking, mix in the 1 cup all-purpose flour, 6 tablespoons cocoa powder, and 1/2 teaspoon salt until just combined with no flour streaks, then spread the batter evenly into the prepared pan.

5-For the swirl, heat the 1/3 cup creamy peanut butter in a microwave-safe bowl for 15 seconds, stir in the 2 tablespoons unsalted butter and 1/4 cup powdered sugar until smooth, and drop spoonfuls of this mixture on top of the batter before swirling it with a knife.

6-Bake for 40 minutes or until a knife inserted in the center comes out mostly clean, then cool for 15 minutes before removing from the pan and cutting into 9 large or 16 small brownies.