Mix milk, sugar, crumbled yeast and a little flour, leave the sourdough to rise. Add the remaining flour, salt and butter. Knead gently the dough and and leave for 30 minutes, covered with food film. Form round buns and leave them for another 10 minutes. Pour 0,5 glass of water in the pot, add some vegetable oil and salt, place buns into the pot and boil under a cover on low heat.