8 ounces (240 mL) strong coffee

½ cup (113 g) unsalted butter, cut into slices

8 ounces (227 g) semi-sweet chocolate, coarsely chopped

¾ cup (150 g) firmly packed light brown sugar

¼ cup (50 g) granulated sugar

3 large eggs, room temperature preferred

1 Tablespoon (15 mL) concentrated coffee

¾ cup (90 g) all-purpose flour

¼ teaspoon salt

2 ounces (40 g) unsweetened baking chocolate, coarsely chopped

4 Tablespoons (57 g) unsalted butter

1 Tablespoon (15 mL) concentrated coffee

1–2 Tablespoons (15–30 mL) milk or cream

2 cups (240 g) powdered sugar

Pinch of salt

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Pour 8 oz strong coffee into a small saucepan, bring to a boil, and reduce to about 2 Tbsp. Remove from heat and set aside.
Preheat oven to 350 °F (177 °C). Line an 8- or 9-inch square pan with foil and spray with nonstick spray.
Melt butter and semi-sweet chocolate together in a saucepan over medium heat until smooth. Let cool for about 15 minutes.
Whisk in light brown sugar and granulated sugar, then eggs one at a time until smooth. Whisk in 1 Tbsp concentrated coffee.
Gently stir in flour and salt just until combined. Spread batter into prepared pan.
Bake for 25–30 minutes or until a toothpick comes out mostly clean. Cool completely before frosting.
For frosting: Melt unsweetened chocolate and butter together over medium heat, then cool 15 minutes.
In a mixing bowl, combine the cooled mixture with 1 Tbsp concentrated coffee and 1 Tbsp milk or cream. On low speed, gradually add powdered sugar, then increase to medium speed and beat until combined, adding salt to taste. Adjust consistency with additional coffee or milk/cream if desired.
Lift cooled brownies from pan using foil edges, place on a cutting surface, and spread frosting evenly. Slice into servings.

You can substitute espresso powder for concentrated coffee for a quicker method.
Ensure brownies are completely cool before frosting to prevent melting.
Store at room temperature for up to 1 week or freeze for up to 3 months.
Adjust frosting thickness by varying coffee or cream amounts.