In a large pan, melt the butter and stir for about 5 minutes, until it starts to brown. Place in the fridge to chill for 30 minutes.
In a large bowl, cream together the sugars, salt, and butter. Whisk in the egg and vanilla, beating and then sift in the flour and bicarbonate of soda, then fold the mixture with a spatula.
Fold in the chocolate chunks, then chill the dough for 30 minutes, or up to overnight (you don’t have to chill the dough, but it does improve the flavour).
Preheat oven to 180°C / 160 C fan.
Scoop the dough with an ice-cream scoop onto a parchment paper-lined baking sheet, leaving at least 4 inches of space between cookies and 2 inches of space from the edges of the pan so that the cookies can spread evenly.
Bake for 10-12 minutes, or until the edges have started to barely brown.
Remove from the oven and sprinkle with flaky salt, then leave to cool.