01 - Preheat oven to 375°F. Grease a standard donut pan thoroughly with nonstick spray or melted butter, ensuring all cavities are coated to prevent sticking.
02 - In a large mixing bowl, sift together all-purpose flour, granulated sugar, baking powder, ground cinnamon, and salt. Whisk until evenly distributed.
03 - In a separate bowl, whisk milk, eggs, melted butter, and vanilla extract until fully incorporated and smooth.
04 - Pour the wet mixture into the dry ingredients. Stir gently with a spatula or wooden spoon just until combined. The batter should remain slightly thick; avoid overmixing to prevent tough donuts.
05 - Transfer batter to a piping bag or use a spoon to distribute evenly into donut pan cavities, filling each approximately three-quarters full.
06 - Bake for 10 to 12 minutes until donuts are lightly golden brown 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 - While donuts are still warm, combine granulated sugar and ground cinnamon in a shallow bowl, mixing thoroughly.
09 - Brush each warm donut lightly with melted butter, then immediately roll in the cinnamon sugar mixture, pressing gently to ensure even coverage on all sides.
10 - Serve immediately while warm for best texture, or allow to cool completely to room temperature.