Preheat the oven to 220°C (200°C fan, gas 7). Separate the eggs and place the egg yolks in a mixing bowl.
Add 4 tbsp warm water and two-thirds of the sugar.
Blitz with a hand blender until pale and creamy.
Whisk the egg whites with the salt, gradually adding the remaining sugar. Whisk until stiff and glossy.
Place the beaten white eggs onto the egg yolk mixture and fold in. Sieve in the flour and cornflour.
Carefully fold in with the spatula.
Pour the mixture onto a baking sheet (30 x 40cm) lined with baking paper. Spread out evenly and bake for 10-12 minutes on the middle shelf of the oven until golden and springy.
Sprinkle a large tea towel with sugar. Carefully turn out the sponge, lightly sprinkle the baking paper with cold water and carefully peel away from the sponge.
Immediately roll up the sponge base tightly, using the tea towel to help, and leave to cool, seam side down.
Carefully unroll the cooled sponge roll. Add 5-6 tbsp of jam and spread out evenly with a spatula.
Whip the cream until stiff. Fold in the remaining jam and spread the cream over the sponge.
Carefully re-roll the sponge using the cloth. Chill for at least 3 hours. Serve with icing sugar.