Preheat oven to 350°F. Grease a standard sized donut pan and set aside.
In a medium bowl, combine flour, sugar, baking soda, baking powder, and salt. In a separate bowl, combine egg, yogurt, vegetable oil, milk, and vanilla extract. Carefully pour the wet ingredients into the dry, mixing until just combined. Mixture will be slightly lumpy and thick.
Spoon batter into a large ziplock bag and cut a ½ inch hole in one corner, creating a makeshift piping bag. Pipe the dough into the donut molds, filling ¾ of the way to the top of the molds. Bake for 10 minutes. Allow to cool in the pan for 5 minutes before coating with cinnamon sugar.
To make the cinnamon sugar, combine both ingredients in a shallow bowl. Using a small brush, brush each donut with the melted butter, then dip each side into the sugar mixture. I dipped each donut in twice for an extra sugary coating 🙂 Serve!