Preheat your oven to 350°F (175°C). Grease your 8x8-inch baking pan or line it with parchment paper for easy removal.
In a double boiler or a microwave-safe bowl, combine the coarsely chopped unsweetened baking chocolate and 1 cup of unsalted butter. Melt them together until smooth, stirring occasionally. Be careful not to overheat the chocolate.
In a large mixing bowl, combine the melted chocolate mixture with 1 ½ cups of granulated sugar and ½ teaspoon of salt. Whisk until well combined. Add in the eggs, one at a time, mixing well after each addition. Stir in 1 teaspoon of vanilla extract.
Sift together 2 tablespoons of Dutch-process cocoa powder and 1 cup of all-purpose flour. Gradually fold this dry mixture into the wet ingredients until just combined. Be careful not to overmix; you want a fudgy texture.
In a separate bowl, mix together ⅓ cup of peanut butter, 2 tablespoons of melted unsalted butter, ½ cup of powdered sugar, and 1 teaspoon of vanilla extract until smooth and creamy.
Pour the brownie batter into the prepared baking pan and spread it evenly. Drop spoonfuls of the peanut butter mixture over the brownie batter. Using a knife or a skewer, gently swirl the peanut butter into the brownie mixture to create a marbled effect.
Bake in the preheated oven for 25-30 minutes or until a toothpick inserted into the center comes out with a few moist crumbs. Do not overbake, as this will make the brownies dry.
Allow the brownies to cool in the pan for at least 15 minutes before lifting them out using the parchment paper. Cut into squares and enjoy your Fudgy Peanut Butter Swirl Brownies!
