Spray donut pan with non-stick cooking spray. Set aside.
In a medium mixing bowl, whisk together flour, sugar, baking powder, and salt.
In a large measuring cup, whisk together buttermilk, egg and oil.
Add the wet ingredients to the dry ingredients and whisk until combined. Do not overmix.
Place the batter in a ziploc bag and close. Snip one corner of the bag.
Pipe the batter out into the prepared pan, filling the donut cavities ⅔ full.
Bake the donuts for 8 to 9 minutes (for me the magic number was 9). The bottom of the donuts should be golden brown and the tops should be light.
Cool the donuts in the pan for 10 minutes. Transfer onto a cooling rack.
Cool completely before coating.
In a shallow dish, mix together sugar and cinnamon.
Place the melted butter in another shallow dish.
Dip each donut (the lighter side) in butter, then in cinnamon sugar. Place back on the rack to cool.
Notes
You can easily double the recipe. You will need ¾ cup of granulated sugar and ¾ cup of buttermilk.
Please note, that the nutrition value can vary depending on what product you use. The information below is an estimate. Always use calorie counter you are familiar with.