Copied from Cristina Galliano’s Spanish Thermomix recipe book and translated by myself.
How to make a quality sponge:
Start by mixing the flour and the baking powder for 30 seconds on speed 3. This is to incorporate air in the mixture, previously we would pass the flour through a sieve held high to incorporate air (at least this is how Delia Smith tells one to do it)
All cakes/sponges are processed by the same method in the Thermomix and the trick for them to come out of the oven very airy and high is put over the butterfly over the blades and beat the sugar and the whole eggs with no temperature for 6 minutes on speed 4 until the mixture takes on a pale colouring. Using no heat makes the batter spongier.
The flour is the last item to be added to the batter and incorporated gently only for 2 seconds on speed 4, to avoid spoiling the airy sponge batter. If 2 seconds is not enough then programme further 1 or 2 seconds (my note: then I would use a whisk and incorporate the flour gently by hand - this is the crucial point in cakes as here all is saved or all is spoiled)
Regards
Barbara