In stand mixer, attach dough hook. On low, add butter, egg yolks, sugar, salt, and cinnamon to liquid. Finally, add flour in 3 batches. Mix on low until dough forms a tight ball. If the dough is sticking to the side of the bowl, add more flour by the tablespoon. NOTE: You don't need a mixer to make these, you can also mix the ingredients in a large bowl with a wooden spoon and then knead lightly on floured surface for a few minutes until dough is soft, but not sticky.