Preheat and prepare: Set your oven to 350°F (175°C). Line a 13×9-inch baking pan with parchment paper, leaving some overhang for easy removal. Lightly grease any exposed sides.
Mix wet ingredients: In a large mixing bowl, whisk together melted butter and cocoa powder until smooth. Add the sugar and mix until well combined.
Incorporate eggs: Add eggs one at a time, whisking well after each addition. The mixture should become smooth and glossy.
Add flavorings: Stir in vanilla extract, espresso powder, and salt, mixing until completely incorporated.
Fold in dry ingredients: Gently fold in the flour and baking powder using a spatula, mixing just until no dry streaks remain. Overmixing can make brownies tough.
Transfer and bake: Pour the batter into your prepared pan, spreading it evenly with a spatula. Bake for 22-25 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. The brownies should still be slightly soft in the center.
Cool briefly: Allow the brownies to cool in the pan for just 5 minutes while you prepare the frosting. Don't let them cool completely—the warm brownies help the frosting spread evenly and create that signature finish.