Preheat your oven to 175°C (350°F). Line or grease a square baking pan (8x8 inches).
In a heatproof bowl, melt dark chocolate and butter together over a double boiler or in the microwave (30-second intervals). Stir until smooth.
Remove from heat and stir in the sugar while the mixture is still warm. This helps give a glossy top to the brownies.
Let it cool slightly, then whisk in the eggs, one at a time, followed by the vanilla extract. Mix until fully combined.
Sift in the flour and salt. Gently fold using a spatula until there are no streaks of flour. Don’t overmix.
Pour the batter into your prepared pan. Spread evenly. Add nuts or chocolate chips on top if desired.
Bake for 20–25 minutes, or until a toothpick comes out with moist crumbs (not wet batter). Let it cool, slice, and enjoy.