Preheat your oven to 350 degrees and spray two donut baking pans with non-stick spray.
In a medium mixing bowl, whisk together the egg and oil. Mix in the buttermilk and vanilla. Add in the flour, sugar, baking powder, and salt and mix until just combined.
The batter will be thin - you can either spoon the batter into the pan or pour it into a piping bag or large zip lock bag, cut the tip off and pipe it into each well, about 3/4 full.
Bake for 10-12 minutes until a toothpick inserted into the center comes out clean. Don't wait until they're golden - you'll over bake them.
While the donuts are baking, mix together the cinnamon and sugar in a large ziplock bag.
Carefully remove the hot donuts from the pan and immediately put them into the cinnamon sugar mixture and shake to fully coat.
Serve warm & enjoy!
If you don't have buttermilk, you can make some by adding 1 Tablespoon of white vinegar OR lemon juice to a measuring cup. Finish filling up 1 cup with milk and let sit for 5-10 minutes to curdle. Done. Buttermilk.
Don't have a donut pan? You can bake these in a muffin pan and cut out the centers or use balls of tin foil or marbles to make the hole before you pour the batter in.
The donuts should easily release from the pan when done baking but if they need help, just run a toothpick along the side of the pan and they should pop out.
You'll want to coat the donuts with the cinnamon and sugar while they're super hot so the mixture sticks.
The donuts are best when served hot and fresh but if you have leftovers, store them in an airtight bag or container for up to a week.