01 - Preheat oven to 375°F. Grease a standard donut pan thoroughly with nonstick spray or melted butter to prevent sticking.
02 - In a large mixing bowl, whisk together all-purpose flour, granulated sugar, baking powder, ground cinnamon, and salt until evenly distributed.
03 - In a separate bowl, whisk milk, eggs, melted butter, and vanilla extract until completely smooth and well blended.
04 - Pour wet mixture into dry ingredients. Stir gently until just combined, being careful not to overmix as this will toughen the donuts.
05 - Transfer batter into a piping bag or use a spoon to distribute evenly into donut cavities, filling each approximately 3/4 full.
06 - Bake for 10 to 12 minutes until donuts are lightly golden and a toothpick inserted into the center comes out clean.
07 - Let donuts rest in the pan for 3 minutes to set, then carefully remove and transfer to a wire rack.
08 - Combine granulated sugar and ground cinnamon in a shallow bowl, mixing thoroughly to create an even coating mixture.
09 - Brush each warm donut lightly with melted butter, then immediately roll in the cinnamon sugar mixture until fully coated on all sides.
10 - Serve immediately while warm for best texture, or allow to cool completely to room temperature before serving.