In a glass measuring cup, heat the milk to 110 degrees F (40 degrees C) and stir in the sugar and yeast. If using active dry yeast, allow it to sit for 15 minutes for the yeast to activate. If using instant yeast, simply move on to the next step.

Mix the eggs and vanilla into the milk yeast mixture.

In the bowl of a stand mixer fitted with a dough hook, mix the flour and salt together. Pour the wet ingredients into the flour and mix on low-medium speed for about 3 minutes, or until it forms a ball around the hook.

Add in a few pieces of butter at a time, allowing them to fully incorporate before adding more. Once all of the butter is incorporated, turn the mixer up to medium-high speed and mix for 8-12 minutes. The dough will eventually pull away from the sides of the bowl and have a silky smooth texture with minimal stickiness. To check if the dough is ready, use the windowpane test - tear off a small piece and carefully spread it out to see if you can see the light through it without it tearing. If it tears, mix for another minute or two and check again.

Once the dough is ready, transfer it to a lightly greased bowl, cover, and allow to proof overnight (or at least 6 hours) in the fridge. Alternatively, you can let the dough rise in a warm place for about 1 hour or until doubled in size.

About 30 minutes before rolling out the dough, fill a dutch oven or heavy bottomed pot with enough neutral oil to cover 2” (5 cm). Place the oil over medium heat and bring to 355 degrees F (180 degrees C).

Place a wire rack over a paper towel lined baking sheet. On a separate baking sheet, cut out 13 4” (10 cm) parchment squares for the donuts.

On a lightly floured surface, roll the dough out to ½” (1.25 cm) in thickness. Brush away any excess flour. Use well floured 3” (7.5 cm) and 1” (2.5 cm) round cutters to cut out as many donuts as possible and place each on a parchment square. Shape the excess dough into a disk, then wrap in plastic wrap and chill for at least 15 minutes before re-rolling and cutting out more donuts.

Cover the donuts and allow to rise for about 30 minutes or until nearly room temperature, but still slightly cool to the touch. When pressed with a finger, the dough will slowly spring back.

Meanwhile, make the cinnamon sugar. In a wide, shallow bowl, combine the sugar and cinnamon. Set aside.

Once proofed, use the parchment squares to gently lower 2-3 donuts into the hot oil. Fry the donuts for 90 seconds on the first side, then flip over and fry for an additional 90 seconds until golden.

Transfer the donuts to the wire rack. Wait a couple minutes for the oil to come back up to 355 degrees F (180 degrees C), then continue frying the donuts.

Once the donuts have cooled just enough to handle, toss them in the cinnamon sugar to coat.