Start by activating dry yeast in a bowl with warm water. Don’t make the water too hot; it can kill the yeast.
Then, in an electric mixer, combine the yeast with flour, milk, butter, egg yolks, sugar, salt, and cinnamon. Mix until a soft dough forms, adding a little more flour if the dough seems too sticky.
Cover the bowl with plastic wrap and a clean kitchen towel and put it in a warm place until the dough has doubled in size, about 1½ to 2 hours.
Dump the dough onto a lightly floured surface and, using a rolling pin, roll to ½-inch thickness. Cut out rounds with a cookie cutter or the rim of a juice glass. Cut smaller holes in the middle, if you’d like. Put the doughnuts on a baking sheet and cover them with a clean kitchen towel, letting them rise until slightly puffed, about 30 minutes.
Then, fry your doughnuts in 350-degree oil, a few at a time, turning them occasionally until they are deeply golden brown. Drain the doughnuts on a paper-towel lined plate and toss them in cinnamon sugar while they are still hot.
Kristie Collado is The Daily Meal’s Cook Editor. Follow her on Twitter @KColladoCook.