Sift flour into a deep mixing bowl. Form a well and fill with crumbled yeast, some milk and sugar. Stir and let rest in a warm spot for approx. 30 minutes. Then add remaining milk, egg yolks, butter, and salt. Mix to form a smooth dough which pulls easily away from sides of bowl. Let rise at least another 15 minutes. Sprinkle work surface with flour and form a roll of dough. Cut into equal pieces. Roll pieces into balls. Let rise. Heat deep-fryer. Using both hands, pull dough apart in the middle until almost transparent, leaving a thick border. Deep-fry until edge of fritter is golden brown and center is still light, turning only once.