Preheat your oven to 350°F (175°C). This ensures your brownies bake evenly and become that fudgy perfection we all love.
Line a 9x13-inch baking pan with parchment paper, allowing some overhang for easy removal later.
In a medium saucepan, melt the 1 cup of butter and the 4 ounces of unsweetened chocolate over low heat. Stir continuously until smooth and combined. Remove from heat and let it cool slightly.
In a large mixing bowl, combine the melted chocolate mixture with 1 2/3 cups granulated sugar. Stir until well blended. Add the 4 large eggs and 1 teaspoon of vanilla extract, whisking until the mixture is smooth and glossy.
In another bowl, whisk together 1 cup all-purpose flour and 1/4 teaspoon salt. Gradually add this dry mixture to the chocolate mixture, folding gently to combine. Be careful not to overmix.
Fold in the 1 cup of peanut butter chips until evenly distributed throughout the batter.
Pour the brownie batter into the prepared pan, spreading it evenly. Bake in the preheated oven for about 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Let the brownies cool completely in the pan on a wire rack.
While the brownies are cooling, start on the first layer of frosting. In a mixing bowl, combine 1 1/2 cups confectioners' sugar, 1/2 cup creamy peanut butter, and 3-4 tablespoons milk. Mix until smooth and creamy. Adjust the milk as needed for desired consistency.
Once the brownies are completely cooled, spread the peanut butter frosting evenly over the top using a spatula.
For the chocolate frosting, beat together 1/4 cup softened butter, 1/4 cup unsweetened cocoa powder, 1/4 cup milk, and 3 cups confectioners' sugar until smooth. Stir in 1 teaspoon of vanilla extract for extra flavor.
Spread the chocolate frosting over the peanut butter frosting layer, smoothing it out as desired.
For best results, refrigerate the brownies for about 30 minutes to set the frosting. Once chilled, lift the brownies out of the pan using the parchment paper overhang. Cut into squares and serve.
