Preheat Oven and Grease Pan:
Preheat the oven to 350°F (175°C) and lightly grease a 12-cavity donut pan or spray with nonstick spray.Whisk Dry Ingredients Together:
Whisk together the dry ingredients (flour, sugar, baking powder, cinnamon, salt) until evenly combined.Whisk Wet Ingredients Together:
Whisk together the wet ingredients (milk, eggs, melted butter, vanilla) until smooth.Fold Wet into Dry:
Fold the wet ingredients into the dry ingredients gently until just combined; avoid overmixing.Fill Donut Pan:
Spoon or pipe the batter into the prepared donut pan, filling each cavity about three-quarters full.Bake Until Golden:
Bake for 10-12 minutes until golden and springy; a toothpick should come out clean.Coat with Cinnamon Sugar:
Brush warm donuts with melted butter and immediately toss in the cinnamon-sugar mixture (1/2 cup sugar + 1 tsp cinnamon) until evenly coated.