In a medium bowl, combine the yeast and lukewarm milk. Add the sugar, yolks, vanilla, cinnamon and whisk until combined.
Add 2 cups and three tablespoons of flour to the liquids. Add the salt and gently stir the salt in the flour to evenly distribute. Roughly mix everything using a wooden spoon or Dutch whisk until a shaggy dough is formed
Dump the dough on your counter or work surface using a bench scraper. Lightly oil your hands and give the dough a rough stir to bring it together—it will be very sticky, and a bench scraper will make your job much easier.
Once you have an evenly mixed but still sticky dough, add the room-temperature butter on top. Squeeze the dough and butter together to incorporate the butter into the dough. Again, it’s going to be sticky.
Once the butter has been incorporated into the dough, sprinkle the remaining 30g (1/4 cup flour) on top of the sticky dough. Lightly oil your hands and start kneading. At first, the dough will feel sticky, but it will slowly become less sticky as you knead.
Knead for 5 to 7 minutes until the dough looks smooth, feels easy to handle, and can easily be shaped into a ball. Initially, lift the dough off the work surface using a bench scraper. Shape the dough into a ball.
Lightly oil the mixing bowl. Add the dough, cover it with plastic wrap, and proof it for 1 1/2 to 2 hours in a warm spot until the dough is visibly larger in size, almost doubled. This will take more or less time, depending on the temperature of your kitchen.