Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan with a bit of unsalted butter and line it with parchment paper, leaving some overhang for easy removal later.
In a medium saucepan over low heat, melt the cubed unsalted butter. Remove it from the heat once fully melted and allow it to cool slightly.
In a large mixing bowl, whisk together the natural cocoa powder and black cocoa powder. This blend gives the brownies their rich, dark color and flavor.
Add the flour, instant espresso powder, and salt to the cocoa mixture. Whisk until combined and set aside.
In another bowl, whisk the eggs until they are frothy. Add the granulated sugar and dark brown sugar, and continue whisking until the mixture is smooth and well combined.
Pour the melted butter into the egg and sugar mixture, stirring until fully incorporated.
Slowly add the dry ingredients to the wet ingredients. Use a spatula to fold the mixture until just combined. Be careful not to overmix; a few flour streaks are okay.
Pour the brownie batter into the prepared baking pan, spreading it evenly. Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Once baked, remove the brownies from the oven and let them cool in the pan for about 10 minutes. Then, lift them out using the parchment paper. Sprinkle the top with flaky sea salt while they are still warm.
Allow the brownies to cool completely before slicing them into squares. Serve them on their own or with a scoop of vanilla ice cream for an extra indulgent treat!
