Preheat your oven to 350°F (175°C) and lightly grease your donut pan or spray with non-stick cooking spray.

In a medium bowl, whisk together the flour, baking powder, granulated sugar, ground cinnamon, and salt.

In another bowl, whisk the milk, egg, melted unsalted butter, and vanilla extract until smooth.

Pour the wet ingredients into the dry ingredients and stir gently with a spatula just until combined. Do not overmix.

Spoon or pipe the batter into the donut cavities, filling each about ¾ full.

Bake for 12-14 minutes, or until the donuts are lightly golden and a toothpick inserted comes out clean.

Let the donuts cool in the pan for 3-5 minutes, then transfer to a cooling rack to cool slightly.

In a shallow bowl, mix the granulated sugar and ground cinnamon for the coating.

Brush each donut with melted butter, then roll in the cinnamon sugar mixture until fully coated.

Serve warm or at room temperature.