I made Nigella's Chocolate Chocolate Chip Muffins from Feast and the recipe and the muffins are lovely, but why do my chocolate chips sink to the bottom of the muffins?
Unfortunately sometimes if chocolate chips are very large they will tend to sink to the bottom of a muffin or cake - this is actually caused by the muffin batter rising up and over the chips as the muffins bake.
If the chips are large then you may like to roughly chop them a little to reduce their size, or you could look for smaller chips. In the UK Silver Spoon chips are fairly small and in the US you can now buy mini chocolate chips. It also helps to toss the chips in a tablepoonful of flour (from the flour used in the recipe) before adding them to the batter. The coating of flour helps the chips to "hold on" to the batter as it rises.