Preheat the oven to 160ºC /325ºF conventional oven. Prepare a 20x20 cm square pan with parchment paper.
In a bowl, sift together the flour, cornstarch and salt.
75 g all-purpose flour, 1 tablespoon cornstarch, ½ teaspoon salt
Melt the butter and chocolate over a double boiler or in the microwave. Add the cocoa powder to the butter/ chocolate mixture and stir it in.
90 g butter, 75 g semi-sweet dark chocolate chips, 35 g unsweetened Dutch cocoa powder
Then add the brown sugar and granulated sugar and with a hand whisk, whisk it together.
130 g granulated sugar, 130 g dark brown sugar
Add the eggs one at a time and the vanilla extract, and mix until incorporated. Once incorporated, continue giving them a good whisk for 1-2 minutes, as this is what will give them the crinkle top.
2 large eggs, 1 ½ teaspoon vanilla extract
Add the dry ingredients to the batter and gently whisk it together until combined.
Pour the brownie batter into the prepared pan, smooth it out with the spatula.
Bake for about 35-37 minutes. Let it cool down completely on a cooling rack.