I tried the Totally Chocolate Chocolate Chip Cookies and the taste was amazing. The only problem is that after cooling down they become really hard. If I warm them up in the microwave briefly, then they get a brownie-like texture which is good. But what can I do to prevent them going hard in the first place.
Nigella's Totally Chocolate Chocolate Chip Cookes (from Nigella Express and on the Nigella website) will firm up on the outside as they cool, but should still remain soft on the inside. If they are hard all the way through then it is most likely that they have been over-baked.
All ovens vary and it may be worth checking your oven with an oven thermometer in case it is running a little too hot. If you have a fan or convection oven then you may need to adjust the temperature and should check your oven handbook for guidelines. Also bake the cookies on the centre rack if you are using a regular oven. if you are using a dark non-stick baking sheet then you may need to reduce the cooking time slightly. The cookies may also be slightly on the small side, which will affect the cooking time required. Nigella shapes her cookies using an ice cream scoop. These usually have a capacity of around 60m/1/4 cup. You should get 12 decent-sized cookies from the dough.
Also make sure that the dough is not over-mixed as this can cause the glutens in the flour to develop too much and become tough. Fold the flour and chocolate chip cookies into the creamed butter mixture until just combined and then shape and bake or freeze the cookies as soon as possible.