Preheat your oven to 177°C (350°F) and line two baking trays with baking paper. Get everything weighted and ready before you start. This batter needs to go unto the oven quickly once it’s made.
Brown the butter. In a saucepan over medium heat, melt the butter then keep cooking and stirring until it turns brown. Watch it carefully so it does not burn.
Remove from heat immediately and add the finely chopped chocolate, neutral oil and espresso powder. Stir until completely smooth and glossy. Set aside to cool for 5 minutes- warm but not hot.
In a separate bowl, whisk together the plain flour, cocoa powder, baking powder, cornstarch/cornflour and salt. Set aside.
This is the most important step. In a large mixing bowl, combine both sugars, the eggs, egg yolk and vanilla extract. Using an eclectic mixer in high speed, beat for a full 5-7 minutes until the mixture is pale, thick and has almost tripled in volume. It should fall from the beater un thick ribbons. Don’t rush this step, it’s what creates that signature shiny, crackly brownie top.
Slowly pour un the warm chocolate mixture and mix until fully combined and glossy.
Add the dry ingredients and fold gently with a spatula until combined. Stop as soon as you can’t see streaks of flour. The batter will be loose and glossy, more like brownie batter than cookie dough. That’s exactly right. If you want to add chocolate chips, you can add it now.
Scoop immediately, using a 2 tbsp cookie scoop, portion the batter onto the tray leaving about 5 cm gap between each cookie. Don’t let the batter sit, bake it straight away for the best crackly top.
Bake for 10-11 minutes. The top should be shiny and crackled, the edges set and the centre still looking slightly underdone. Pull them out at this point.
Sprinkle with flaky sea salt while warm. Leave to cool on the tray before moving them.
Video: