Brew Coffee: Pour the brewed coffee into a small saucepan and heat over medium heat until boiling. Reduce the coffee down to 2 tablespoons. Remove from heat and set aside.
Coffee Brownies:
Preheat the oven to 350ºF (177ºC). Line an 8″ or 9″ square baking pan with foil and spray with nonstick spray. Set aside.
In a medium saucepan over medium heat, melt the butter and chopped chocolate, stirring frequently until completely smooth. Allow to cool for about 15 minutes.
Whisk the light brown sugar and granulated sugar into the cooled chocolate mixture until combined. Add the eggs one at a time, whisking after each addition until smooth. Whisk in 1 Tablespoon of the concentrated coffee.
Gently fold in the flour and salt until completely combined.
Pour the batter into the prepared pan and bake for 25-30 minutes, or until a toothpick inserted in the center comes out mostly clean. Allow the brownies to cool completely before frosting.
Mocha Frosting:
In a small saucepan over medium heat, melt the unsweetened baking chocolate and butter. Allow to cool for 15 minutes.
In a medium bowl, using a handheld mixer or a stand mixer with a paddle attachment, combine the cooled chocolate/butter mixture, concentrated coffee, and 1 tablespoon of milk or cream. Gradually add the powdered sugar, increasing the mixer speed to medium until the frosting is smooth and well combined. Add more milk or cream if needed for the desired consistency. Add a pinch of salt to taste.
Assemble: Once the brownies are completely cooled, lift them from the pan using the foil overhang. Spread the frosting evenly over the top, then cut into individual servings.
Store leftover brownies at room temperature for up to 1 week. Brownies also freeze well for up to 3 months. Thaw in the refrigerator overnight.