Preheat the oven to 350F (180 Celcius) degrees.
Line a 9x13 inch (23 x 33 cm) metal pan with parchment paper, leaving an overhang around the sides.
Add the butter and chopped chocolate to a large heatproof bowl and microwave for 1-minute intervals on medium power, stirring between each, until melted and smooth.
Remove from the microwave and whisk in the sugar.
Whisk in the eggs 1 at a time, followed by the vanilla extract.
Gently fold in the cocoa powder, flour and salt.
Pour the batter into the prepared pan and bake for 32-37 minutes, or until an inserted toothpick comes out clean or with a few damp crumbs.
Cool brownies completely before frosting