In a small bowl, whisk together flour, baking powder, salt, and sugar.
In another bowl, whisk milk, egg, and melted butter. Combine wet and dry ingredients until just mixed (do not overmix).
Heat oil in a deep pot over medium heat until 350°F (175°C).
Using a small cookie scoop or spoon, drop spoonfuls of batter into hot oil. Fry for 2–3 minutes, turning to brown evenly.
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Remove with a slotted spoon and drain on paper towels.
In a shallow bowl, mix cinnamon and sugar. While warm, roll donut holes in the mixture until well coated.
Serve immediately and enjoy!