Preheat the oven to 350°F (180°C) and line a 9x13 inch baking pan with parchment paper.
Melt the butter and dark chocolate together in a heatproof bowl over simmering water or in the microwave, stirring until smooth.
In a separate bowl, whisk the eggs, sugar, and vanilla extract until light and fluffy.
Gently fold in the melted chocolate mixture.
Sift in the flour and salt, then fold gently until just combined.
Pour the batter into the prepared pan and spread evenly.
Bake for 20-25 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Allow to cool completely in the pan before slicing into 24 squares.