Preheat and Prepare: First, preheat your oven to 180°C (160°C fan). Generously grease a 23-25cm (9-10 inch) cast-iron skillet with a little unsalted butter, ensuring you cover the bottom and sides well.

Brown the Butter: Place the 170g of 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 5-8 minutes, the sizzling will subside, and you'll see golden-brown specks forming at the bottom. It will smell intensely nutty. Immediately remove it from the heat and pour it into a large mixing bowl to cool for 5-10 minutes. For a deep dive into the science, Serious Eats has a great guide on how to brown butter.

Mix the Sugars: To the bowl with the slightly cooled browned butter, add the light brown sugar and caster sugar. Whisk vigorously for about a minute until the mixture is smooth and well combined.

Add Eggs and Vanilla: Add the egg, egg yolk, and vanilla extract to the butter mixture. Whisk again until everything is fully incorporated and the mixture looks glossy.

Combine Dry Ingredients: In a separate, smaller bowl, whisk together the plain flour, bicarbonate of soda, and fine sea salt. Add this to the wet ingredients. Using a spatula or wooden spoon, fold the dry ingredients in until just combined. Be careful not to overmix; stop when you no longer see streaks of flour.

Fold in the Chocolate: Gently fold in your chopped dark chocolate chunks, distributing them evenly throughout the dough.

Assemble the Skillet Cookie: Scrape the cookie dough into your prepared cast-iron skillet. What works best for me is using the back of a spoon or slightly damp hands to press the dough into an even layer across the bottom of the skillet.

Bake to Perfection: Place the skillet in the preheated oven and bake for 20-25 minutes. The edges should be set and golden brown, but the centre should still look slightly soft and underdone. This is key for a gooey middle.

Rest and Serve: Carefully remove the skillet from the oven and place it on a heatproof surface. If you like, sprinkle over a little flaky sea salt. Let the pan cookie rest for at least 10-15 minutes. This allows the centre to set up a little more. Serve warm, straight from the skillet, with scoops of vanilla ice cream.