Preheat and Prepare: First, preheat your oven to 180°C (160°C fan). Place a 23-25cm (9-10 inch) cast-iron skillet inside the oven to heat up while you prepare the dough. This helps create a wonderfully crisp base.

Brown the Butter: Place the unsalted butter in a light-coloured saucepan over a medium heat. Melt the butter, then continue to cook it, swirling the pan occasionally. It will foam and sizzle. After 5-7 minutes, the sizzling will subside, and you'll see golden-brown specks at the bottom of the pan. It will smell intensely nutty. Immediately remove it from the heat and pour it into a large mixing bowl to cool for 5 minutes. Learning to properly brown butter is a fantastic skill for any home cook.

Mix the Wet Ingredients: Add the light brown sugar and caster sugar to the bowl with the cooled browned butter. Whisk vigorously for about a minute until well combined. Add the egg, egg yolk, and vanilla extract, and whisk again until the mixture is smooth and glossy.

Combine the Dry Ingredients: In a separate bowl, whisk together the plain flour, bicarbonate of soda, and salt.

Form the Dough: Add the dry ingredients to the wet ingredients. Using a spatula, fold everything together until just combined. It's important not to overmix at this stage; stop as soon as you no longer see streaks of flour. What works best for me is to fold gently to keep the cookie tender.

Add Chocolate: Gently fold in most of the dark and milk chocolate chips, reserving a handful for the top.

Assemble the Skillet Cookie: Carefully remove the hot skillet from the oven. Add the remaining 1 tablespoon of butter to the skillet and swirl it around to coat the bottom and sides. Scrape the cookie dough into the hot skillet and use your spatula to spread it evenly.

Bake to Perfection: Scatter the reserved chocolate chips over the top of the dough and sprinkle generously with flaky sea salt. Place the skillet back in the oven and bake for 20-25 minutes. The edges should be golden brown and set, but the centre should still look slightly soft and underdone.

Rest and Serve: Let the skillet cookie cool on a wire rack for at least 15 minutes before serving. This allows the centre to set up a little more. Serve warm, straight from the skillet, with scoops of vanilla ice cream.