I'm really struggling to find decent quality dark chocolate chips anywhere in London, can you recommend anywhere or should I give up? Will cutting up a bar of good chocolate and adding that do instead? I am worried about them just melting and making a mess rather than staying as chocolate morsels within the baked cookie.
Usually chocolate chips in a cookie may melt slightly while a cookie is baking, but it will not mix in with the baking cookie dough and the chocolate will re-solidify as the cookie cools, leaving distinct morsels of chocolate still in the cookie (please see our recent Kitchen Query on this topic).
We have made Nigella's cookies with a variety of chocolate chips and chunks, including Toll House Dark Chocolate Morsels, Silver Spoon Dark Chocolate Chips, Waitrose Dark Chocolate Chunks and Ghiradelli Bittersweet Chocolate Chips (in the US). The cocoa solids content of these chocolate chips tends to be slightly lower than the 70% dark chocolate bars, but most people find that this is fine for baking in cookies. Hotel Chocolat have various locations in London and sell chocolate drops with 70% cocoa solids that you could try. Jane Asher sells a very high quality chocolate chip with 70% cocoa solids (Callebaut) and these may be worth trying if you are close to her store in London (they are also available by mail order) however you may find these are slightly large for your cookies and more frequently these chips would be used for melting.
It is possible to cut up a good quality chocolate bar, if you prefer. You will need a heavy, sharp knife to do this and it is easier to cut up the chocolate if it is at room temperature. Cut the bar into 2-3mm chunks (roughly 1/16 inch). You will find that you get some shards of chocolate that are too small to use in cookies and these can be scooped up and saved in an airtight container to be sprinkled over ice cream, cakes or cupcakes, or used for melting.