In a large mixing bowl, whisk together the flour, sugar, baking powder, salt, and ground cinnamon.

In a separate bowl, whisk together the milk, eggs, melted butter, and vanilla extract until well combined.

Gradually add the wet ingredients to the dry ingredients, stirring until just combined. Be careful not to overmix; the batter should be slightly lumpy.

In a deep saucepan or deep fryer, heat about 2 inches of vegetable oil to 350°F (175°C). Use a thermometer to maintain the temperature for consistent frying.

Using a small cookie scoop or a tablespoon, carefully drop spoonfuls of batter into the hot oil. Fry in batches to avoid overcrowding. Fry the donut holes for about 2-3 minutes, turning them occasionally until they are golden brown and cooked through. Use a slotted spoon to transfer the donut holes to a paper towel-lined plate.

While the donut holes are still warm, toss them in a mixture of cinnamon and sugar until evenly coated.

Serve warm and enjoy the delicious homemade donut holes.