01 - Preheat oven to 375°F. Grease a standard donut pan thoroughly with nonstick spray or butter.
02 - Whisk together flour, sugar, baking powder, cinnamon, and salt in a large bowl until evenly distributed.
03 - Whisk milk, eggs, melted butter, and vanilla extract in a separate bowl until completely blended and smooth.
04 - Pour wet ingredients into dry ingredients. Stir gently until just combined; avoid overmixing to keep donuts tender.
05 - Transfer batter to a piping bag or spoon directly into prepared donut cavities, filling each about 3/4 full.
06 - Bake for 10-12 minutes until lightly golden and a toothpick inserted into the center comes out clean.
07 - Let donuts rest in pan for 3 minutes to set, then carefully transfer to a wire rack.
08 - Mix granulated sugar and cinnamon in a shallow bowl until thoroughly combined.
09 - Brush each warm donut lightly with melted butter, then roll in cinnamon sugar mixture until evenly coated on all sides.
10 - Serve immediately while warm or allow to cool to room temperature before serving.