Start by browning the butter. In a small pan, place the chopped butter. Gently melt on a medium heat. Stirring constantly, the butter will begin to foam and sizzle slightly around the edges. In approx 5-8 minutes from when you started, the butter will start to turn golden brown. Some foam will subside and the milk solids at the bottom of the pan will be toasty brown. The butter in the pan will smell intensely buttery and nutty.
Now remove the pan from the heat and carefully transfer the brown butter to a mixing bowl and set aside to cool for 5 minutes.
In another mixing bowl, add the flour, bicarbonate of soda and the cornflour. Whisk together to combine fully.
In the bowl with the melted butter, add both types of sugar, and whisk until incorporated. Now add the whole egg and egg yolk, along with the vanilla extract. Whisking again until combined.
Now fold through the dry ingredients until a soft dough begins to form. Finally, stir through the chopped chocolate. Cover the bowl containing the cookie dough and chill in the fridge for at least 2 hours or up to 3 days. I usually chill mine overnight. After overnight chilling, I leave my cookie dough at room temperature for 15 minutes before rolling.
When ready to bake, divide the cookie dough and roll into 60g portions. Preheat oven to 170°C / 150°C Fan / 325°/ Gas Mark 3. Line a couple of baking trays with parchment paper and place the cookies about 3-inches apart.
Bake the cookies for 12-14 minutes until crisp and lightly golden around the edges, but still soft in the centre. Place any additional chocolate chips/chunks on the tops of cookies if desired. I also like to sprinkle the tops with some flaky sea salt.
Cool the cookies on the baking trays for 10 minutes, before enjoying. The cookies will keep stored at room temperature in an airtight container for up to 3 days.