Sprinkle the yeast over warm milk and dissolve well.
Whisk in ½ cup of the flour until you have a creamy paste. Let it rise in a warm place until the dough has risen and fallen, approximately 1 hour.
If making the dough by hand: add the eggs, sugar, cinnamon, vanilla, salt and the remaining flour to the fallen yeast mixture. Combine well and add the butter. Knead the dough until the consistency is smooth and elastic. Add additional flour if the dough is too sticky. Add additional milk if it’s too dry.
If making the dough with a food processor or mixer: combine the remaining flour, sugar, salt and cinnamon in the food processor, pulse until well mixed or use a dough hook for the mixer. Add the butter and mix. Add the 4 eggs and mix. Add the yeast mixture and the vanilla (and any other flavorings). Mix until the dough start to form a large ball. If the texture is too sticky then add some more flour. If it’s on the dry side, add milk (a couple tablespoons at a time).
Remove the dough from the food processor and form a large ball, place it in a large bowl.
Let the dough rise in a warm place, covered with a kitchen towel or plastic wrap, until the size doubles, about 2 hours.
Punch the dough down and work the dough a few times.