Preheat oven to 170°C (340°F). Grease and line a small baking tin.
Melt butter & chocolate: In a heatproof bowl, melt butter and chocolate over a double boiler or microwave in 20-second bursts.
Whisk eggs & sugar: In a separate bowl, whisk eggs, granulated sugar, and brown sugar until pale and slightly thickened.
Combine: Stir the melted and slightly cooled chocolate mixture into the eggs.
Add dry ingredients: Sift in flour, cocoa powder, and salt. Fold gently.
Bake: Pour into the prepared tin and bake for 35-40 minutes, until just set.
Cool & serve: Let brownies cool before cutting.