Whisk together flour, sugar, cornstarch, baking powder, cream of tartar, salt and cinnamon.
In separate bowl, whisk together milk, egg melted butter and vanilla extract. Gradually stir the wet ingredients into the dry until just combined.
Pour the batter into a large ziploc bag and snip off the corner of the bag for a makeshift piping bag.
Pipe the batter into the donut pans, filling each mold 2/3 of the way full.
Bake mini donuts for 8 minutes and regular sized donuts for 10-11 minutes.
Meanwhile, melt 1/4 cup butter in a shallow bowl.
Combine 1/2 cup sugar and 2 tsp cinnamon in another shallow dish.
When donuts are done baking, allow to cool at least 5 minutes and until you are able to handle them with your hands. Dunk each donut in butter on both sides (or brush with butter using a pastry brush) and then press in cinnamon sugar mixture on both sides.
Transfer to cooling rack and serve.
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