Bring coffee to a simmer in a saucepan over medium-low heat and reduce to 2/3 cup of a thick coffee concentrate, stirring constantly to avoid burning, about 30 minutes. Set aside and let cool to room temperature.
Preheat oven to 350 degrees F (175 degrees C). Grease an 8x8-inch baking pan.
Stir sugar and melted butter together in a bowl until smooth; beat in eggs, followed by vanilla extract. Stir the coffee concentrate and cocoa powder into the wet ingredients until no lumps remain. Whisk flour, salt, and baking powder together until thoroughly combined in a separate bowl; stir flour mixture into wet ingredients to make a smooth batter. Spread batter into the prepared baking pan.
Bake in the preheated oven until a toothpick inserted into the center of the brownies comes out with moist crumbs, 30 to 35 minutes. Let cool and cut into bars.
