Yogurt Cloud Cake: The softest cake ever!

Separate the yolks from the egg whites.
mix the grated lemon zest, yogurt and sifted flour with the egg yolks and mix well with a whisk.
add the lemon juice to the egg whites (at room temperature) and start beating. As soon as foam forms on the surface, add 1/3 of the sugar and continue to beat three times. You should get a smooth, whipped mixture.
add the egg whites three times to the egg yolk mixture and mix gently from bottom to top.

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