Do not forget that all products for pastry should be warm, not lower than 20-25 degrees. Sift the flour.
Heat the milk to t 25-28 degrees Celsius, dilute the yeast in it, half the sugar norm and 3 tbsp flour. Mix well and put in a warm place for 40-50 minutes.
If your room is cool, but there is a microwave oven, you can do this - in a thick-walled glass or glass bowl, heat the water in the microwave until it boils, at this time the walls of the oven are well heated and steam accumulates, turn off the heat, take out the dishes with water, and send a bowl with sourdough, close the oven door (a very convenient option when it is cool).
After the yeast has come to life and the sourdough has come up, add the rest of the sugar, salt, sour cream and flavoring. Separate one yolk from the eggs, cover it and set aside, mix the rest well and pour into the sourdough. Mix thoroughly until the sugar and salt are dissolved, add the flour (leave a little). Knead with a spoon or spatula, then add warm melted butter (you can also cream, but with melted muffin tastier and more flavorful). The dough should be elastic, soft, but in any case not spread out. This largely depends on the quality of the flour, if the dough does not hold its shape, knead a little more flour.