Preheat the oven to 400°F. Coat a 6-cavity donut pan with nonstick cooking spray.
In a large bowl whisk together the flour, baking powder, baking soda, and salt. In medium bowl stir together the egg, yogurt, vinegar, oil, and maple syrup.
Make a hole in the center of the dry ingredients and pour the wet ingredients into the hole. Gently fold everything together until combined. The batter will be thick.
Scoop batter into a piping bag or ziploc bag, snip a bottom corner with scissors, and squeeze out the batter evenly into a donut pan.
Bake for 10 minutes or until golden brown. Let cool for a few moments and then flip over onto a cooling rack while still warm.
In a small shallow bowl, combine the sugar and cinnamon.
Use a pastry brush the melted butter over the donuts then dip each donut in the cinnamon sugar mixture to coat. Serve soon after coating with cinnamon sugar mixture for best texture.