Preheat your oven to 350°F (175°C). This ensures that your brownies bake evenly and achieve that perfect fudgy texture.
In a double boiler or a microwave-safe bowl, melt the 6 ounces of chopped semi-sweet or dark chocolate together with 3/4 cup of unsalted butter. Stir until smooth and let it cool slightly.
In a large mixing bowl, combine 1 cup of granulated sugar and 1/2 cup of packed light brown sugar. Add the 2 tablespoons of espresso or brewed coffee and 1 teaspoon of vanilla extract. Mix until combined.
Add the 3 large eggs to the sugar mixture, one at a time, stirring well after each addition. This will help create a smooth, glossy batter.
Pour the melted chocolate and butter mixture into the egg and sugar mixture. Stir until fully incorporated.
Gently fold in the 1 cup of all-purpose flour and a pinch of salt (if using). Be careful not to overmix; you want to keep that fudgy texture!
Pour the brownie batter into a greased 9x9 inch baking pan, spreading it out evenly. Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Once baked, remove the brownies from the oven and let them cool in the pan on a wire rack. This step is crucial for achieving the perfect fudgy texture.
In a large mixing bowl, beat together 1 cup of creamy peanut butter and 1/2 cup of softened unsalted butter until smooth. Add 2 teaspoons of vanilla extract and gradually mix in 1 to 2 cups of confectioners sugar. Adjust the sugar to achieve your desired sweetness and consistency.
Once the brownies are completely cooled, spread the peanut butter buttercream evenly on top. Feel free to add sprinkles for a fun touch!