Preheat your oven to 350°F (175°C). Line a baking pan (9x9 inches works perfectly) with parchment paper or grease it with butter to ensure the brownies come out easily.

In a microwave-safe bowl, combine the chopped dark chocolate and melted butter. Heat in 30-second intervals, stirring after each, until fully melted. Be patient! Stirring prevents the chocolate from burning. Once melted, let it cool slightly.

In a large mixing bowl, whisk together the eggs, granulated sugar, and brewed espresso. The mixture should be smooth and light in color. Add the melted chocolate and butter, followed by the vanilla extract, sea salt, and mix until combined.

In a separate bowl, sift together the all-purpose flour and unsweetened cocoa powder. Gradually fold this dry mixture into the wet mixture. Be careful not to overmix; this may create dense brownies. Stir until just combined.

If you’re using walnuts, fold them into the brownie batter now. They bring a delightful crunch.

Pour the brownie batter into the prepared pan, spreading it evenly. Bake in the preheated oven for 20-25 minutes. Keep an eye on them — the edges should be set, while the center remains slightly soft. You can do the toothpick test; it should come out with a few moist crumbs.

Once baked, remove the pan from the oven and let it cool on a wire rack for at least 30 minutes. After cooling, lift the brownies out of the pan using the parchment paper. Cut them into squares, and voilà!