Preheat oven to 350°F. Lightly butter a 9x9-inch metal baking pan and set aside.
In a small saucepan, melt the baking chocolate and butter over low heat until completely melted. Let cool slightly.
4 ounces semisweet baking chocolate, ½ cup unsalted butter
Pour the melted chocolate into a mixing bowl and use a hand mixer to mix in the granulated sugar, eggs, vanilla extract, and sea salt.
¾ cup granulated sugar, 2 large eggs, 1 teaspoon pure vanilla extract, ¼ teaspoon fine sea salt
Stir in the flour and cocoa powder.
1 cup all-purpose flour, 3 tablespoons unsweetened cocoa powder
Spread the brownie batter into the prepared pan.
Place the peanut butter in a microwave-safe bowl and microwave for 30-60 seconds, or until it becomes runny.
⅓ cup creamy peanut butter
Drizzle the peanut butter over the brownie batter and use a skewer or butter knife to swirl the peanut butter and brownie batter.
Bake for 25-28 minutes, or until the middle is no longer jiggly. Allow to cool completely in the pan before cutting into squares and removing.