This recipe is not my own, and in fact I'm probably breaching copyright by putting it up here at all! It's taken from a small booklet called "The Mazola method for perfect baking" which came free with, you guessed it, Mazola corn oil, about a million years ago (c. 1970s)
Line bottom of two 7" sandwich tins with greaseproof paper and grease lightly. Sieve dry ingredients into a bowl. Separate yolks from whites of eggs. Mix corn oil, yolks and water together lightly with a fork. Stir the mixture into the dry ingredients and beat well to form smooth, slack batter. Whisk egg whites until stiff and fold lightly into mixture. Turn into tins and bake in moderately hot oven (375'F. Mark 5) for 25-30 minutes. Cool on wire tray. Sandwich together with jam and dust with caster sugar or top with icing.
All these notes are my own. They're mostly aimed at people who lack confidence in cake-making and/or have limited cooking facilities, equipment or budgets.
This recipe is really practically unbreakable. To be honest, most cake recipes are a lot more robust than people seem to think; you generally don't have to stick to the letter of the recipe. For example:
You don't have to use Mazola Corn Oil; that bit's just a marketing gimmick I've used bog-standard cheap vegetable oil, supermarket own-brand corn oil, real Mazola, and Mazola 'lite', and never noticed any difference.
I often don't bother to sieve the dry ingredients, and at the moment I don't even own a sieve (although I did have to shove the cocoa for the chocolate version through a tea-strainer because it was so lumpy!).
The process of 'folding lightly' (which can scare novice cooks) is less fragile than people seem to think, and will cope with a fair amount of inexpert folding (trust me on this) -- basically, as long as you don't beat it or mix too hard, it'll be fine. The point of it is to keep the air in the whisked egg whites -- this is what makes the sponge light and fluffy. The worst that will happen if you do it badly is that your sponge will be a bit heavy.
The cooking time is by no means exact and will largely depend on your oven. (By the way, that 375°F is equal to 190°C.) For fan-assisted ovens you're usually advised to reduce the cooking temperature by 10 degrees (this is a general rule, not just for this recipe -- apologies if this is really obvious). Basically, though, cook it until it's done (see below).
How to tell if cakes like this are cooked: stick a sharp knife or a skewer or something in it, right in the middle because that's where it's likely to be thickest (and therefore take longest to cook). If the knife comes out clean the cake's probably done; if the knife comes out sticky it probably needs a bit longer.
Apparently if you open the oven too often while it's cooking, a cake can collapse. I've never had this happen to me and I'm often terribly impatient and keep opening the oven to look. Probably not worth worrying about unduly, to be honest. :-) (I only mention it because it was something I used to be really worried about!)
Obviously, you don't have to make a sandwich sponge cake -- the recipe does just as well for a solid sponge. Actually, this isn't obvious, I remember it occurring to me that it might not cook properly if all that cake mix was in one tin. So let me assure you that it does work (but takes a bit longer to cook), and even works fine with double quantities, although cooking time will need adjusting (takes about an hour in my oven). Double quantities will be about right for a 10" square cake. I've never tried more than double, probably not advisable since even double-quantities is getting dangerously near to burning on top by the time it cooks in the middle.
Separating eggs is slightly tricky, but comes with practice (and is made completely trivial if you have an egg-separator, but I'll assume people haven't since most people don't seem to). Crack the egg carefully, just enough to get your nail into the shell and split it in half. The aim is to keep the yolk in the shell and pour the white out (I find transferring the yolk from shell to shell to get the bits underneath it out helps). A bit of egg-white getting in the yolks won't do any harm; but a bit of yolk in the whites will make it very difficult to whip the whites until they're stiff. It takes ages if you're just using a fork or an egg-beater, although it's possible -- a rotary-whisk is useful here (but not a blender, they over-beat the eggs and don't get enough air in -- or so I find at any rate), but it still takes a while and you really want the whites to be stiff enough to stay in peaks when you lift a bit up. This is what makes the sponge light. I suspect a sponge cake with only partially-stiffened egg whites would be edible but a bit flatter and heavier.
Baking parchment (or greaseproof paper, which is cheaper and just as effective) really does help -- I grease both the inside of the tin and the inside of the paper with margarine. The latter stops the cake sticking to the paper (it's a bastard to peel off sometimes!) and the former makes the paper stick to the tin so you can pour the cake mix in without the paper falling in! However if you don't have baking parchment or greaseproof, it's still do-able; just make sure the cake tin is really well greased, and when it's cooked slip a knife round the side before trying to get the cake out.
I don't remember ever having tried the coffee or citrus versions, so if you try either please let me know how well it works... The chocolate one is great though, and cocoa is much easier to get hold of from e.g. the corner shop than cornflour is, as I found out to my irritation when realising that none of the three shops within quick cycling distance of my flat sold cornflour!