Brown the butter (the soul of the flavour): Place the butter in a medium, light-coloured saucepan over medium heat. Swirl the pan occasionally as it melts. You'll hear it begin to crackle and pop as the water evaporates. Watch for the milk solids to turn a deep golden-amber colour and smell intensely nutty and toasty—this takes about 5-7 minutes. Immediately pour it into a large, heatproof bowl to stop it from burning. Let it cool for 10 minutes until it is warm but not hot to the touch.

Cream the sugars and butter: Add both sugars to the warm browned butter. Beat with an electric mixer on medium speed for 2-3 minutes. The mixture will look like wet sand and feel slightly grainy against the bowl. It should smell deeply of caramel.

Add the wet ingredients: Beat in the whole egg, the egg yolk, and the vanilla extract. Continue mixing for a full 2 minutes until the batter is pale, thick, and glossy. When you lift the beaters, the mixture should fall in a thick, ribbon-like stream.

Combine the dry ingredients: In a separate bowl, whisk together the flour, cornflour, bicarbonate of soda, and fine sea salt. The clumps will disappear, and the colour will be a uniform, pale beige.

Fold everything together: Add the dry ingredients to the wet mixture in two additions, mixing on low speed until just combined. You should see a few streaks of flour remaining. Now, fold in the chopped milk chocolate chunks with a rubber spatula. The dough will be stiff and slightly sticky, studded with large, uneven pieces of chocolate.

Chill the dough (non-negotiable): Cover the bowl with cling film and refrigerate for at least 45 minutes, or up to 2 hours. The dough will firm up significantly, becoming less sticky and much easier to handle.

Shape and bake: Preheat your oven to 180°C (160°C fan). Line two large baking trays with parchment paper. Scoop the chilled dough into 8-10 large balls, about 80g each. Place them on the trays, leaving 8-10cm of space between each. Do not flatten them. Bake one tray at a time for 12-14 minutes. The cookies will spread, their edges will turn a deep golden brown, and the centres will look puffed, pale, and slightly underdone. The smell of browned butter and melted chocolate will fill your kitchen.

Finish and cool: As soon as you remove the tray from the oven, sprinkle each cookie with a pinch of flaky sea salt. Let them cool on the tray for 10 minutes. The residual heat will continue to set the centres. After 10 minutes, transfer them to a wire rack to cool completely, if you can wait that long.