01 - Preheat oven to 375°F. Grease standard donut pan thoroughly with nonstick spray or melted butter.
02 - Sift flour, sugar, baking powder, cinnamon, and salt into large mixing bowl. Whisk until evenly distributed.
03 - Whisk milk, eggs, melted butter, and vanilla extract in separate bowl until fully incorporated and smooth.
04 - Pour wet mixture into dry ingredients. Fold gently until just combined, taking care not to overmix.
05 - Transfer batter to piping bag or zip-top bag. Pipe or spoon batter into prepared pan, filling each cavity three-quarters full.
06 - Bake 10-12 minutes until lightly golden and toothpick inserted into center comes out clean.
07 - Let donuts rest in pan 3 minutes. Carefully remove and transfer to wire rack.
08 - Combine granulated sugar and ground cinnamon in shallow bowl. Mix thoroughly.
09 - Brush each warm donut lightly with melted butter. Roll in cinnamon sugar mixture, pressing gently to adhere coating evenly.
10 - Serve immediately while warm, or allow to cool to room temperature.