1. Sift together the flour and bicarbonate of soda into a large mixing bowl, add the salt and set aside.
2. Cream together the softened butter, caster and soft light brown sugar in a stand mixer for 3–5 minutes until pale and very soft. Use a rubber spatula to scrape down the sides of the mixing bowl, add the vanilla and mix again. Add the beaten eggs, a little at a time and beat until fully incorporated.
3. Add the sifted dry ingredients and mix again to combine. Finally, fold in the chopped chocolate using a large rubber spatula or metal spoon. At this point you can bake the cookies immediately but I find that their flavour develops better if you cover and chill the cookie dough in the fridge for 24 hours.
4. Bring the dough to room temperature at least 30 minutes before baking. Preheat the oven to 170°C (325°F) Gas 3.
5. Spoon rounded tablespoons of cookie dough onto the prepared baking sheets allowing plenty of space between each. Bake in batches on the middle shelf of the preheated oven for 10–12 minutes, until the edges of the cookies are golden brown and the middle is still slightly soft. You may need to turn the sheets around halfway through baking to ensure that the cookies brown evenly.
6. Allow the cookies to cool on the baking sheets for 3–4 minutes then transfer to wire racks to cool before serving. Store the cookies in an airtight container lined with baking parchment for 4–5 days.