Dissolve the yeast in 150 ml (approximately 1/2 cup) of lukewarm water and stir until smooth.
Mash the feta cheese in a bowl with a fork. Cut the butter into pieces.
Mix everything with an electric hand mixer until creamy, add the flour and the dissolved yeast and knead.
Knead the dough on a floured surface for a few minutes, adding extra water if too dry. Cover and let rise in a warm place for about 45 minutes. Grease a baking sheet with butter and preheat the oven to 200°C (approximately 400°F).
Knead the dough on the floured surface, divide into about 32 equal portions and shape them into balls. Whisk the yolks with 1 tablespoon of water and place the balls on the greased baking sheet.
Bake on the middle rack of the oven until golden brown for about 25 minutes. Serve lukewarm.