Combine Wet Ingredients: In a large bowl or the bowl of a stand mixer, add the cooled melted butter, light brown soft sugar, and granulated sugar. Whisk or beat on a medium speed for about 2-3 minutes until the mixture is well combined and has a glossy, wet sand appearance.

Add Eggs and Vanilla: Add the whole egg, egg yolk, and vanilla extract. Beat again for another minute until the mixture becomes lighter in colour and texture. It's important to scrape down the sides of the bowl to ensure everything is incorporated.

Combine Dry Ingredients: In a separate bowl, whisk together the plain flour, bicarbonate of soda, baking powder, and sea salt flakes. This step ensures the leavening agents are evenly distributed, which helps the cookies rise uniformly.

Mix Dough: Gradually add the dry ingredients to the wet ingredients, mixing on a low speed until just combined. Be careful not to overmix here; stop as soon as you no longer see streaks of flour. Overmixing can develop the gluten too much, leading to a tough cookie.

Fold in Chocolate: Gently fold in the chopped dark and milk chocolate chunks using a spatula. I like to reserve a handful to press onto the tops of the cookie dough balls before baking.

Chill the Dough: Cover the bowl with cling film and refrigerate for at least 1 hour, or up to 72 hours. The longer it chills, the more flavourful the cookies will be. This step is crucial for thick, chewy cookies.

Preheat and Prepare: When you're ready to bake, preheat your oven to 180°C (160°C fan). Line two large baking trays with parchment paper.

Shape and Bake: Scoop out balls of dough, roughly 2 tablespoons each. An ice cream scoop works wonderfully for uniform sizing. Arrange them on the prepared trays, leaving at least 5cm of space between each one to allow for spreading. If you reserved some chocolate, press a few chunks onto the top of each ball.

Bake to Perfection: Bake for 12-15 minutes, or until the edges are golden brown and the centres look slightly underdone. What works best for me is rotating the trays halfway through baking for an even colour.

Cool and Enjoy: Let the cookies cool on the baking tray for 5 minutes before transferring them to a wire rack to cool completely. They will firm up as they cool. A sprinkle of extra sea salt flakes on top of the warm cookies is a fantastic final touch.