01 - Preheat oven to 375°F. Grease a standard donut pan thoroughly with nonstick spray or butter to prevent sticking.
02 - Sift together flour, sugar, baking powder, cinnamon, and salt in a large mixing bowl. Whisk until uniformly blended.
03 - Whisk milk, eggs, melted butter, and vanilla extract in a separate bowl until completely smooth and emulsified.
04 - Pour wet mixture into dry ingredients. Fold gently with a spatula until just combined—some small lumps should remain. Avoid overmixing to prevent tough donuts.
05 - Transfer batter to a piping bag or zip-top bag with corner snipped. Pipe batter into prepared pan, filling each cavity approximately three-quarters full.
06 - Bake for 10 to 12 minutes until donuts spring back when touched lightly and a toothpick inserted into the thickest part emerges clean. Tops should be pale golden.
07 - Let donuts rest in pan for 3 minutes to set. Carefully invert pan to release donuts onto a wire rack. Continue cooling until warm but still manageable to handle.
08 - While donuts cool, combine granulated sugar and ground cinnamon in a shallow bowl. Mix thoroughly to distribute cinnamon evenly.
09 - Brush each warm donut generously with melted butter using a pastry brush. Immediately press buttered side into cinnamon sugar mixture, coating thoroughly. Repeat on reverse side.
10 - Transfer coated donuts to a clean wire rack or serving plate. Serve while still warm for optimal texture and flavor, or allow to cool completely to room temperature.