1. Preheat your oven to 350°F (175°C) and lightly grease a donut pan with butter or non-stick spray.
2. In a large mixing bowl, combine the flour, granulated sugar, baking powder, salt, and cinnamon. Stir until well blended.
3. In a separate bowl, whisk together the melted butter, milk, egg, and vanilla extract until smooth.
4. Gradually add the wet ingredients to the dry ingredients, stirring gently until just combined. Be careful not to overmix, as this can lead to tough donuts.
5. Spoon the batter into the prepared donut pan, filling each mold about three-quarters full.
6. Bake for 12-15 minutes, or until the donuts are lightly golden and spring back when touched.
7. Remove from the oven and let cool in the pan for a few minutes before transferring to a wire rack.
8. Once cooled, dust the donuts with powdered sugar for a light, sweet finish.

These donuts are truly a delight to make, and nothing beats the satisfaction of biting into a freshly baked treat straight from the oven.