01 - Preheat oven to 375°F and grease a standard donut pan with nonstick spray or butter.
02 - In a large bowl, whisk together flour, sugar, baking powder, cinnamon, and salt until well blended.
03 - In a separate bowl, whisk together milk, eggs, melted butter, and vanilla extract until smooth and combined.
04 - Pour the wet mixture into the dry ingredients and stir gently until just incorporated; take care not to overmix.
05 - Spoon or pipe batter evenly into the prepared donut pan, filling each cavity about three-quarters full.
06 - Bake for 10 to 12 minutes until donuts are lightly golden and a toothpick inserted comes out clean.
07 - Let donuts rest in the pan for 3 minutes, then carefully transfer to a wire rack.
08 - Combine sugar and cinnamon in a shallow bowl, stirring until evenly distributed.
09 - Brush each warm donut lightly with melted butter, then roll in the cinnamon sugar mixture until thoroughly coated.
10 - Serve immediately while warm or allow to cool to room temperature before serving.