Heat your oven to 180C/350F/Gas 4. Grease and line the base of a 23cm/9in springform tin.
Put the eggs and sugar in a heatproof bowl and place over a pan of simmering water (don't let the bowl touch the water). Use an electric whisk to whisk them together until thick, pale and moussey and doubled in volume. The mixture should hold a trail when you lift the beaters out of the mixture. Take off the heat.
Fold the melted butter gently into the egg mousse, then add in the flour and one packet of strawberry angel delight then fold in the mixture until fully incorporated.
Pour into the prepared tin and bake for 15–20 minutes, or until cooked and lightly golden-brown. Leave to cool completely, then remove the sponge from the tin and peel off the parchment paper.