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 flour, sugar, baking powder, cinnamon, and salt until evenly distributed.
03 - In a separate bowl, whisk milk, eggs, melted butter, and vanilla extract until completely smooth and incorporated.
04 - Pour wet ingredients into dry ingredients. Stir gently until just combined; the batter should remain slightly thick with visible flour streaks. Avoid overmixing.
05 - Transfer batter to a piping bag or zip-top bag with corner snipped. Pipe batter into prepared donut cavities, filling each approximately three-quarters full.
06 - Bake for 10 to 12 minutes until donuts are lightly golden and a wooden toothpick inserted into the thickest part emerges clean.
07 - Let donuts rest in the pan for 3 minutes to set. Carefully remove from pan and transfer to a wire rack.
08 - While donuts cool slightly, combine granulated sugar and ground cinnamon in a shallow bowl, stirring until uniform.
09 - Working with one donut at a time, brush generously with melted butter, then immediately roll in cinnamon sugar mixture, pressing gently to adhere coating evenly.
10 - Serve immediately while warm for best texture, or allow to cool completely. Store leftovers in an airtight container at room temperature for up to 2 days.