01 - In a large bowl, whisk together flour, sugar, baking powder, baking soda, salt, and cinnamon.
02 - In a separate bowl, whisk together milk, eggs, melted butter, and vanilla extract.
03 - Pour the wet ingredients into the dry ingredients and mix until just combined; do not overmix.
04 - Heat vegetable oil in a deep pot or Dutch oven to 350°F (175°C).
05 - Using a small cookie scoop or two spoons, carefully drop tablespoon-sized balls of dough into the hot oil. Fry in batches, turning occasionally, until golden brown and cooked through, about 2 to 3 minutes per batch.
06 - Remove donut holes with a slotted spoon and drain on a paper towel-lined plate.
07 - While still warm, brush donut holes with melted butter, then roll in the cinnamon sugar mixture until fully coated.
08 - Serve donut holes warm or at room temperature.