Preheat your oven to 350°F (175°C) and grease a 9×13 inch baking pan.

In a medium saucepan, melt the butter and dark chocolate together over medium heat, stirring until smooth.

Remove from heat and whisk in the cocoa powder and sugar until well combined.

Add the eggs one at a time, whisking well after each addition, then stir in the vanilla extract.

Fold in the flour and salt until just combined.

Pour the batter into the prepared baking pan, spreading it evenly.

Bake for 25-30 minutes or until a toothpick inserted into the center comes out with a few moist crumbs.

Allow to cool in the pan before cutting into squares and serving.