Preheat and Prepare: Place a 10-inch (25cm) cast iron skillet into your oven and preheat it to 180°C (160°C fan). Getting the skillet hot first is key to a great crust.

Brown the Butter: In a small, light-coloured saucepan, melt the 170g of unsalted butter over a medium heat. Continue to cook, swirling the pan occasionally, for about 5-8 minutes. The butter will foam, then the foam will subside, and you'll see golden-brown specks forming at the bottom. It will smell intensely nutty. As soon as you see these specks, remove it from the heat and pour it into a large mixing bowl to stop the cooking process. Understanding the science behind this, often called the Maillard reaction, can really improve your baking.

Mix the Wet Ingredients: Let the browned butter cool for a few minutes. Then, add the 200g of light brown sugar and 50g of caster sugar to the bowl and whisk until well combined. Add the egg, egg yolk, and 2 teaspoons of vanilla extract, and whisk vigorously until the mixture is smooth and slightly lightened in colour.

Combine the Dry Ingredients: In a separate bowl, whisk together the 225g of plain flour, 1/2 teaspoon of bicarbonate of soda, 1/2 teaspoon of baking powder, and 1/2 teaspoon of fine sea salt.

Make the Dough: Add the dry ingredients to the wet ingredients and fold them together with a spatula until just combined. Be careful not to overmix. I find that stopping when there are still a few streaks of flour is best.

Add the Chocolate: Gently fold in the 200g of chopped dark chocolate, distributing it evenly throughout the dough.

Bake the Skillet Cookie: Carefully remove the hot skillet from the oven. Add the cookie dough to the centre of the skillet and use your spatula to quickly spread it out to the edges in an even layer. Sprinkle the top with a little flaky sea salt, if using.

Final Bake: 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 underdone and soft. It will continue to cook in the hot skillet after you remove it.

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 properly. Serve warm, straight from the skillet, ideally with a scoop of vanilla ice cream.