In a saucepan, melt 225 g of unsalted butter over medium heat. Stir constantly for about 5 minutes or until the melted butter turns golden brown and releases a nutty aroma. Once done, remove from heat and let it cool for 45 minutes.

In a large mixing bowl, combine the cooled brown butter with 200 g of brown sugar and 100 g of caster sugar. Beat in 1 large room temperature egg and 1 egg yolk, followed by 1 tsp of vanilla extract. Mix until the mixture is smooth and creamy.

In a separate bowl, whisk together 300 g of plain flour, 3 tsp of cornflour, 0.25 tsp of baking powder, 0.5 tsp of baking soda, and 0.5 tsp of cooking salt.

Gradually incorporate the dry ingredient mixture into the wet ingredients, stirring just until combined. Fold in 250 g of dark chocolate chips and 150 g of milk chocolate chips evenly throughout the dough.

Divide the dough into eight equal portions, roll each portion into a ball, then flatten them into discs about 3.75 cm thick.

Refrigerate the shaped cookie discs for 12 to 24 hours.

Preheat your oven to 180°C (350°F). Line a baking tray with parchment paper. Bake for approximately 17 minutes, or until the edges are golden brown.

Allow the cookies to cool on the baking tray for about 20 minutes to firm up before transferring them to a wire rack.