Preheat the oven to 350 degrees Fahrenheit.
Grease two 6 cavity donut pans (if you only have one, don't worry, you'll just need to do 2 rounds since this recipe makes 8). Set aside. In a large mixing bowl, whisk together all dry ingredients. Be sure to spoon and level the dry ingredients into the measuring cups. They should just naturally fall in, do not pack them in!
Whisk in the wet ingredients until a smooth batter forms.
Add about 1/4 cup worth of batter to each donut cavity, it will depend on your pan, but 1/4 cup is generally what I use. You should have about 1/4-1/8 of an inch between the batter and the top of the pan.
Add to the oven and bake for 12-15 minutes. My oven is perfect at 14. They should be rounded at the top and a toothpick should come out clean. A few crumbs is ok, but no wet batter.
Remove from the oven and let cool in the pan for about 10 minutes.
Flip over onto a cooling rack, they should fall right out and let cool another 5-10 minutes.
Melt the butter in a shallow bowl and whisk together the sugar and cinnamon in a second bowl.
Dip each donut very lightly in the butter, or brush on top, then dip in the cinnamon sugar. I flip it around in the sugar quite a few times to make sure it is well coated, and spin it around on the side to really get it on there!
Transfer back to the cooling rack or a plate and enjoy!