It's a sort of sponge cake cooked in a very specific way, to ensure that it is light and flexible enough to roll without breaking. You will need an electric whisk for this.
Measure out 125g of caster sugar into a basin, pudding bowl or other heatproof vessel. Crack in 3 eggs and get out your whisk. Boil a saucepan of shallow water with a low trivet in and place the bowl over the top. Whisk and whisk and whisk until the mixture is pale and extremely light and fluffy looking, almost overflowing the bowl.
Remove from the heat and whisk for a bit longer, letting it cool down a bit. Measure out 125g of plain flour, seive it into the eggy mixture and gently fold it in, followed by a tablespoon of hot water.
Line a shallow baking tray with greaseproof paper, sprinkled with a little sugar, and pour the mixture in. Bake in the oven for 10-15 minutes at 200 degrees, depending on the thickness of your baking tray. Check on the poor thing: don't let it burn!
Lay a damp teatowel onto your worksurface with another sheet of greaseproof paper on top, again sprinkled with a little sugar. When the cake is done, quickly upend the cake onto the paper on the teatowel. Peel the used paper off the bottom of the cake (now the top).
Heat up some your favourite flavour jam in a saucepan, this will make it nice and easy to spread. Spoon or pour it onto the cake and use a knife to spread it all over. Since my Dad is diabetic, technically he shouldn't eat any swiss roll at all. However, that would be what happens in fantasy land, in the real world he will inhale it until the whole family shout at him, so I have used a bare scraping of jam to try to minimise the damage.
Roll the cake up towards you, using the teatowel to help grip it firmly and taking care not to break the sponge. The centre should be tightly rolled, the outer layers less so. Leave to cool, if you can, before eating slices off the two ends. The rest, at least in our house, belongs to the trifle.
