First of all, weigh your eggs and try to remember the number - this is the amount of margarine, sugar and flour that you will need.
Preheat your oven to 180°C (or 160°C fan).
Cream together the margarine and sugar until they're light coloured and fluffy - I use my lovely electric stand mixer for this, which makes it easy, but you can do the same with a wooden spoon and some frantic beating. This is probably the most important bit and makes for a nice light sponge, so it's worth putting in a bit of time/elbow grease at this stage.
Add the lightly beaten eggs, one at a time and beat in quickly until combined - you don't need to spend too long on this - it's better not to overbeat. If the mixture looks like it's curdling, then add a few tablespoons of the flour between eggs.
Add about a tablespoon of the coffee and beat in quickly.
Put in the flour and salt (and baking powder if applicable) into the bowl and fold in with a spatula or metal spoon - again, you don't want to spend too much time here - just the minimum amount of mixing until everything is combined.
Spread the mixture gently into greased sandwich cake tins (this also works well as a flat slab cake in lined brownie style tin, though you obviously sacrifice the buttercream filling that way) and stick in the oven for about 25 minutes or until a knife or skewer poked into the middle of the cake comes out clean. You might want to turn them once half way through, but, other than that, it's generally best to avoid opening the oven door too much - particularly near to the beginning.
Remove the cakes from the oven and leave on a rack to cool.
When the cakes are cold enough to handle (and not to disintegrate), remove them from the tins and leave to cool completely.
For the icing, beat the margarine with the icing sugar until it's sort of crumbly - the stand mixer or an electric whisk makes this pretty easy, but, again, a wooden spoon and some hard work will also do the job.
Add the rest of the coffee (and a smidgen of milk if it looks a bit dry) and beat fast and crazily until the icing is as fluffy as you can possibly make it (I usually do about 5 minutes on a fast speed in the stand mixer). You have to keep an eye on this, though, as it can go a bit gloopy if you beat it for too long.
Recipe courtesy of Jennie Botherston, All the Things I Eat