Preheat oven to 375°F (190°C) and grease a donut pan or baking sheet.

In a large bowl, whisk together flour, sugar, baking powder, salt, and cinnamon.

In a separate bowl, whisk milk, melted butter, and egg until smooth.

Add wet ingredients to dry ingredients and mix until just combined.

Spoon or pipe batter into prepared donut pan, filling each cavity about 3/4 full.

Bake for 10–12 minutes until lightly golden and a toothpick inserted comes out clean.

Allow donuts to cool slightly while preparing the coating.

Mix cinnamon and sugar in a shallow bowl.

Dip warm donuts in melted butter, then roll in cinnamon sugar until fully coated.

Serve warm or allow to cool completely before storing.