01 - Preheat oven to 375°F. Grease standard donut pan with nonstick spray or butter.
02 - Whisk together flour, sugar, baking powder, cinnamon, and salt in a large bowl until well blended.
03 - Whisk milk, eggs, melted butter, and vanilla extract in separate bowl until fully combined.
04 - Pour wet ingredients into dry ingredients. Stir until just combined; avoid overmixing to prevent tough donuts.
05 - Spoon or pipe batter into prepared donut pan cavities, filling each approximately 3/4 full.
06 - Bake for 10-12 minutes until lightly golden and toothpick inserted comes out clean.
07 - Let donuts cool in pan for 3 minutes, then carefully transfer to wire rack.
08 - Combine sugar and cinnamon in shallow bowl for coating mixture.
09 - Brush warm donuts with melted butter, then roll in cinnamon sugar until evenly coated.
10 - Serve immediately while warm or allow to cool to room temperature.