Add the chocolate, butter and sugar to a cooking pot or sauce pan. Mix this on a low heat until all the ingredients are melted.
In the meanwhile, you can sieve the flour and cornstarch in order to get the lumps out.
Pour the chocolate mix into another bowl. Mix it with an electric handmixer until the mixture is cooled down to room temperature.
Add the eggs, one by one, while whisking. Once incorporated, you can add the salt, vanilla and flours. Mix this until you have an even batter without any lumps.
Pour the batter in the baking tin. To remove all the bubbles in the batter, you can gently tap the baking tin on the kitchen counter.
Bake the brownies for about 25-30 minutes.