Which cocoa powder do you recommend for the Devil's Food Cake? Also I want to make a 14'' square by 4'' high cake using Nigella's recipe,how much do you reckon I will need to achieve this? Thanks for your time.
We would suggest using a cocoa powder from a quality chocolate maker and there are several different ones around. Green & Black's is available in many countries and Valrhona is also widely distributed. In the US Ghiradelli is another good brand (sometimes this is sold as "baking cocoa").
What is important is that the cocoa powder is unsweetened, so do not confuse it with hot chocolate or drinking cocoas, which may have sugar and vegetable fats added.
The UK does not differentiate between "natural" cocoa and "dutch processed" cocoa (which has been treated to reduce the cocoa's acidity). In this recipe either type will work as the raising agents are a combination of baking powder and bicarbonate of soda (baking soda). If only bicarbonate of soda is used in a recipe then it would be best to use the natural form of cocoa as its acidity is needed to react with the bicarbonate of soda so that the cake rises.
Unfortunately it is quite difficult to scale this type of cake so we regret we are unable to give guidelines on quantities. Additionally a 14-inch/35cm square sponge cake is a very large size to attempt in a domestic oven. We would suggest consulting a specialist cake decorating book as sometimes these have scaled recipes which are suitable for larger pan sizes.