1. Heat the oven to hot, 220°C, gas mark 7.
2. Grease and line a Swiss roll tin 20 X 30 cm.
3. Whisk the eggs and sugar together in a bowl over a pan of hot water, taking care that the base does not touch the water.
4. Whisk for 10-15 minutes until thick and creamy.
5. Remove from the pan and whisk until cold.
6. Sift the baking powder with the flour and salt, and fold in lightly.
7. Pour into the prepared tin and bake for 10 minutes.
8. Meanwhile, warm the jam, if used.
9. When the cake is cooked, turn it on to a large sheet of greaseproof paper dusted with caster sugar.
10. Peel off the lining paper.
11. Trim off any crisp edges.
12. Spread the cake with the warmed jam and roll up tightly.
13. Dredge with caster sugar and place on a cooling rack with the cut edge underneath.
Note If the Swiss roll is to be filled with butter cream, cover with greaseproof paper and roll up tightly with the paper inside.
Cool completely. When cold, unroll carefully, spread with the filling, and re-roll. Dust with caster sugar.