Whisk the egg whites with an electric mixer until soft peaks (around 3 to 4 minutes)Separate the egg whites from the yolks and beat the egg whites until stiff then add the caster sugar and whisk until stiff peaks form (around 1 to 2 minutes).
In another bowl, whisk the egg yolks with the remaining sugar until the mixture is light and fluffy. Add the grated lemon rind, the starch, the almond flour and the yogurt and stir until all the ingredients are well blended.