01 - Preheat oven to 375°F. Grease a standard donut pan thoroughly with nonstick cooking spray or softened butter.
02 - In a large mixing bowl, whisk together the flour, sugar, baking powder, cinnamon, and salt until evenly distributed.
03 - In a separate bowl, whisk the milk, eggs, melted butter, and vanilla extract until completely smooth and incorporated.
04 - Pour the wet mixture into the dry ingredients. Stir gently with a spatula until just combined; avoid overmixing to prevent tough donuts.
05 - Transfer the batter to a piping bag or use a spoon to distribute it evenly among the donut cavities, filling each about 3/4 full.
06 - Bake for 10 to 12 minutes until the donuts are lightly golden and a toothpick inserted into the center comes out clean.
07 - Let the donuts rest in the pan for 3 minutes to set, then carefully remove them and transfer to a wire rack.
08 - While donuts cool slightly, combine the coating sugar and cinnamon in a shallow bowl, mixing thoroughly.
09 - Brush each warm donut generously with melted butter, then immediately roll in the cinnamon sugar mixture until completely coated.
10 - Serve the donuts warm for the best texture and flavor, or allow them to reach room temperature before serving.