Cream the butter, dark brown sugar and white sugar in a bowl. Once pale and fluffy, crack the egg in and add vanilla if desired. Sift in your self-raising flour and salt.
Fold in the flour and salt just until you can't see any streaks. Then, fold in chocolate.
Cover the bowl with Saran wrap and rest in the fridge for at least an hour or up to 2 days at most. (If freezing dough, dough can be frozen for upto 2 months).
Preheat the oven to 375°F.
Scoop the dough into tablespoon-sized balls and place onto a baking tray with 2-inch spaces in between the cookies..
Bake for 7 minutes for a chewy cookie, or 10 minutes for a crispy cookie.
Let cool and serve, or enjoy warm with a glass of cold milk.