I just love Nigella's videos. I have tried quite a lot of her recipes and they were all delicious. So now I tried her Spruced-Up Vanilla Cake (Christmas, p199) and I don't know what happened , but when I mixed the ingredients in a food processor it wasn't creamy consistency. Instead it curdled on me. I put it in the oven anyway and it was delicious, but why did the mixture curdle - maybe I used too much butter? Thanks and always a fan.
The Spruced-Up Vanilla Cake has quite a high ratio of eggs to other ingredients - Nigella mentions in the introduction how it can be called "Eggy Vanilla Cake" and this will tend to make the batter susceptible to curdling. It is most likely that the eggs were fridge-cold when they were added - next time try allowing them to come to room temperature before using them as eggs and butter will tend to blend better if they are at similar temperatures.
The yogurt in the mixture will also sometimes result in a lttle curdling as it is adding a liquid to a fat (yogurt to butter) and again it will help a bit if the yogurt is not straight out of the refrigerator. As you have noted the baked cake will still be fine as long as the batter is only slightly curdled and hs not completely split (with the fat separating from the liquid completely). Creamed cakes that curdle sometimes rise a little less but as the Spruced-Up cake is made using an all-in-one method and is a fairly liquid batter, most the rise comes from the egg and the bicarbonate of soda (baking soda)so the curdling should not affect the final texture of the cake.