Butter creamed with white and brown sugar, then flour and a generous two cups of chopped chocolate. The two sugars are what give them crisp edges and a chewy middle. Makes about twelve.
Two sugars, two jobs. White sugar spreads and crisps; brown sugar holds moisture and keeps the middle chewy. Using only one gives a cookie that is either uniformly crisp or uniformly soft. Take them out while the centres still look underdone: they carry on cooking on the hot tray, and a cookie that looks finished in the oven is dry by the time it cools. Chopped chocolate from a bar is better than chips here, because chips are made to hold their shape while chopped chocolate melts into seams through the dough.