I recently bought a carton of eggs and was delighted when I cracked open the first egg to find two yolks. Delight turned into confusion when I was 4 eggs and 8 yolks into my carton. Since the recipe (Nigella's Spruced Up Vanilla Cake from Christmas, p198) calls for 6 eggs, I was unsure how to continue. I ended up including two more egg whites. Is there a recommended way for dealing with double-yolked eggs in baking? Or should they just be treated as regular eggs? After all, their weights are the same.
Whilst double yolked eggs are quite rare in supermarket eggs (due to the eggs being checked under strong light, known as "candling", for quality control purposes) they are sometimes found in Farmers' Market eggs, farmstand eggs and sometimes are sold specifically as double yolked eggs. Double yolks are supposed to be more common in younger hens that haven't fully established their laying cycle and as the hens get older they will tend to settle into laying single yolked eggs. This may explain a full carton of double yolked eggs as one would assume they all came from a fairly new flock of laying hens.
Double yolked eggs can in most cases be used on a one-for-one basis for regular eggs in baking. You will find that the cakes will be richer as the yolks contain more fats but, as in using duck eggs for baking, this would be considered a positive by most people. However, for the Spruced Up Vanilla Cake it may have been wise to offset the richness of the eggs with a couple of extra whites as the recipe itself is quite rich with eggs (this is noted in the introduction) and it is always useful to store any left over egg whites in the freezer to use up on an occasion such as this. If the cake has a high proportion of eggs, or uses whisked egg whites in the recipe, then unfortunately it would be better if possible to try and use another carton of eggs or make up the difference with extra egg whites. If this happens again it may be worth trying to buy another carton of eggs for baking and saving the double yolked ones for breakfast as they make impressive fried eggs and very rich scrambled eggs or omelettes.