Chocolate Chip Madeleines

"This recipe is (very slightly) adapted from the second St. John restaurant cookbook, Beyond Nose to Tail (if you don't already have both the St. John books, buy them now as not only are the recipes brilliant but the turn of phrase is a joy). I would recommend you buy one or two 12-hole madeleine pans for this, if you don't already have one, as once you've made these you'll be sure to make them again. You can serve the first batch while the second batch is in the oven; you'll need both...."