1. Brown the butter: Place the butter in a frying pan or small saucepan and melt it over medium heat. Continue cooking, stirring frequently. The butter will initially foam, and then small brown specks will begin to appear at the bottom of the pan.
2. Once the butter develops a deep golden colour and the brown specks appear, after approximately 5 minutes, remove it from the heat immediately. At this stage, you have the French beurre noisette—brown butter with a distinctive nutty aroma reminiscent of hazelnuts.
3. Pour the brown butter into a large glass bowl, including all the brown bits from the bottom of the pan. Leave it to cool for 15–20 minutes.
4. Add the brown sugar and white sugar and mix thoroughly with a whisk.
5. Add the egg, egg yolks and vanilla, and whisk until the mixture is smooth and glossy.
6. In a separate bowl, combine the flour, baking powder, baking soda and salt. Gradually add the dry ingredients to the butter mixture, folding with a spatula until just combined. Do not overmix.
7. Add both types of chocolate and fold them gently into the dough. Cover the bowl and refrigerate the dough for approximately 2 hours.
8. Preheat the oven to 180°C on the fan setting and line a baking tray with non-stick baking paper.
9. Using an ice cream scoop, portion out approximately 75 g of dough and shape it into a ball. Do not press or flatten it. Place it on the prepared tray, leaving plenty of space between each ball of cookie dough.
10. Bake for approximately 10–12 minutes, until the edges are set but the centre is still soft and looks slightly underbaked.
11. Remove the tray from the oven and leave the cookies on the tray for 10 minutes without moving them. As they cool, they will set while remaining soft and chewy inside.

Note: The unbaked cookie dough balls can be stored in the freezer. When ready to bake, place them directly from the freezer onto the baking tray and bake in the preheated oven for approximately 2–3 minutes longer.