Once the chilled brown butter has turned solid, remove from the fridge and beat it with a hand-held or stand mixer for about a minute or so until it looks smooth and creamy.
Next, beat in the sugar (on medium high speed) until mixed well. The beaten butter and sugar mixture should be light in colour with a fluffy texture.
Then beat in the egg, egg yolk and vanilla (medium high speed) until well combined with a pale colour and creamy consistency.
Sift in the flour in a a separate bowl. Add the cornflour, baking soda and salt and whisk it together so that everything is evenly combined.
Fold in the dry ingredients into the wet mixture until just combined. Don't overmix!
Gently fold in the chocolate chips.
Cover/ seal the bowl tightly (I use cling-film) and chill for at least a few hours (but preferably for 12-24 hours).
When you remove the dough, let it come to room temperature. Pre-heat the oven to 175°C and prep two baking trays with parchment paper, greasing well.
Roll out the dough into balls using a cookie/ ice cream scoop (you want about 1.5 tbsp of dough for each ball).
Bake the cookies for 11 – 12 minutes until the edges start to turn golden-ish brown but the centre is still soft.
After 5 – 10 minutes, transfer the cookies carefully on to a wire rack and let them completely cool before reaching for your first (then second and third!) one. A cold glass of milk on the side is optional but recommended!