Prepare yeast dough for baking. To do this, mix kefir and water, heat the mixture to a pleasantly warm temperature.
Add yeast and sugar. Mix everything, cover and leave in a warm place for 15-20 minutes or until foam caps form.
Separately mix 500 g of flour, vanilla and salt.
In a small bowl with a fork, lightly whisk an egg at room temperature into a soft mousse.