Preheat the oven to 350°F and grease the donut pans, unless you're using silicone donut pans.
Mix together the milk and vinegar in a small bowl and let sit for at least 5 minutes.
Stir together all of the dry ingredients for the donuts.
Add in all of the wet donut ingredients and stir well.
Pour, or spoon, about ¼ cup of the batter into each donut cavity. The batter should fill the donut cavities about ⅔ of the way up; if it doesn’t, adjust it until it does.
Bake the donuts for about 13 minutes, using a toothpick to check when they are done. Allow the donuts to cool for at least 10 minutes on a wire rack before topping them.
Make the topping by whipping together the cinnamon and sugar.
Once the donuts are cool, dip them in the melted butter, then cover the donuts in the cinnamon and sugar mixture by dipping, rolling, or sprinkling the mixture over the donuts.