For the Brioche Dough:

1. Heat the buttermilk to 110 degrees F and stir in yeast and sugar. If using active dry yeast, allow to sit for 15 minutes. If using instant yeast, simply move on to the next step.
2. Mix the eggs and vanilla into the milk yeast mixture.
3. Combine flour, malted milk powder, and salt together in the bowl of a stand mixer fitted with a dough hook. 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.
4. Add in one piece of butter at a time, allowing it to fully incorporate before adding the next. Once all of the butter is incorporated, turn the mixer up to medium-high speed and mix for 8-10 minutes. To check if the dough is ready, 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 and check again.
5. Once the dough is ready, transfer it to a lightly greased bowl, cover, and allow to proof in a warm environment for about one hour or until doubled in size. Alternatively, you can proof the dough overnight in the fridge.

For Frying the Donuts:

1. Roll the dough out to 1/2" in thickness. Use one large and one small round cutter to cut out as many donuts as you can. Cover them with plastic wrap as you re-roll and cut out the remaining dough. Allow the donuts to proof at room temperature for 30 minutes.
2. While the donuts proof, add enough vegetable or canola oil in a cast iron pot to fill two inches and heat to about 360-370 degrees F. The temperature of the oil will drop about 5 degrees when you add the donuts, so you want to keep it around 365 degrees. 
3. Fry the donuts, two or three at a time, for 30 seconds on each side for a total of about 2-3 minutes (about 1 minute for the donut holes). Dry donuts off on paper towels quickly and then immediately toss in the bowl of cinnamon sugar. Transfer to a cooling rack over a baking sheet to cool completely.