In a large bowl, whisk together the flour, sugar, baking powder, baking soda, salt, and cinnamon until well combined.
In another bowl, beat the eggs, then add the buttermilk and vanilla extract. Mix until smooth.
Gradually add the wet ingredients to the dry ingredients, stirring until just combined. Do not overmix; a few lumps are okay.
Heat about 2 inches of vegetable oil in a deep skillet or pot over medium heat until it reaches 350°F (175°C).
Using a piping bag or a squeeze bottle, carefully pipe the batter into the hot oil in a spiral or circular shape, about 4 inches in diameter. Fry for about 2-3 minutes on each side or until golden brown.
Remove the donuts from the oil and place them on a paper towel-lined plate to drain excess oil.
Dust the warm donuts with powdered sugar before serving.