Preheat your oven to 350°F (175°C) to ensure even baking.
Line a 9x13-inch baking pan with parchment paper, leaving some overhang for easy removal.
In a medium saucepan over low heat, melt the chopped dark chocolate and unsalted butter, stirring gently. Add the instant espresso powder and mix well. Remove from heat and let cool slightly.
In a large mixing bowl, whisk together the granulated sugar, brown sugar, and eggs until well combined. Stir in the vanilla extract.
Pour the melted chocolate mixture into the egg and sugar mixture and stir until fully incorporated. In a separate bowl, whisk together the flour, salt, and cocoa powder. Gradually fold the dry ingredients into the wet mixture until just combined.
Pour the brownie batter into the prepared baking pan, smoothing it into an even layer. Bake for 25-30 minutes, or until a toothpick comes out with a few moist crumbs.
Allow the brownies to cool in the pan on a wire rack for about 20 minutes. Use the parchment overhang to lift them out and let cool completely before cutting into squares.
