Check the yeast: (see footnotes if you'd like to make this dough in the breadmaker)
Dissolve 2 tsp of active dried yeast with 1/2 cup milk and leave in a warm location until frothy, about 10-15 minutes. If the mixture does not froth up and rise, discard the yeast and start over when you have a fresh batch of dried yeast.
Into a bowl of a mixer sift 3 cups of flour. Then add 1/3 cup sugar, remaining 1/2 cup warm milk, 4 egg yolks, 2 tbsp sour cream and the risen yeast mixture. Mix with a paddle attachment on low speed until it comes together, then switch to a kneading hook and knead for about 20 minutes or until the dough is elastic, tacky to touch and you're able to stretch a piece of dough into a thin "window without it tearing.
Add the butter and the salt and knead it into the bread until no streaks of butter are seen. Sometimes tearing the dough into chunks helps to incorporate the butter easier.
Oil the sides of the bowl and the dough, cover the bowl with a kitchen towel and leave in a warm place to proof until doubled in size, about 1.5 hours. I preheat the oven at 200F for 2-3 minutes then turn it off and place the bowl into the oven with the heat off. This gives the dough a nice warm place without being too hot. Be careful not to overheat the oven, as anything over the temp of 100F will kill the yeast instead of allowing it to proof.
This recipe makes one large braid. To make two smaller bread loaves divide the dough into two, then shape 2 separate braids instead of one like the recipe instructs to.
Once risen, divide the dough into 3 equal pieces. Weighing the dough is the best way to divide the dough evenly. Roll each piece into a tight ball, then into a 20-inch log. Pinch the three strands together, then braid them into a braid pinching the three ends together at the end as well. Tuck both ends of the braid underneath itself slightly to give it a better shape.
Transfer to a 12"x18" baking sheet, cover with a clean tea towel and transfer to a warm place once again until the braid doubles in size. You can repeat the trick with preheating the oven slightly once again.
Once doubled in size remove from the oven and preheat the oven to 350F with the baking rack in the middle.
Brush the braid with a beaten egg yolk and sprinkle with sesame seeds or poppy seeds if you wish.
Bake the braid for about 30-35 minutes or until the top is golden in color and/or the inside of the sweet bread registers 195F. If the top starts to brown before the inside registers 195F loosely tent the bread with foil.
Once baked, remove from the oven and allow to cool completely before cutting or eating.
If you'd rather make the dough in the bread maker add the ingredients in the following order: milk, butter, salt, egg yolks, sour cream, flour, sugar, yeast. Turn the bread maker to a 1.5 hour dough cycle and allow it to run through. Then, continue with steps 4 and beyond.