Preheat the oven to 350ºF or 180ºC and spray a donut pan with non-stick spray or grease it with some oil. Set aside.
Combine the dry ingredients in a large mixing bowl (flour, sugar, baking powder, ground cinnamon, and salt).
Add all the remaining ingredients (flax egg, melted vegan butter, vanilla extract, and milk) and stir until well combined.
Fill each pan cavity approximately 3/4 full. You can spoon the batter into the donut pan (it can get messy), or fill a ziplock bag with the batter, cut a hole in the corner, and pipe it into the pan.
Bake for 15 to 20 minutes or until lightly golden brown and a toothpick comes out clean.
Remove from the oven and allow the donuts to cool for at least 5 minutes before removing them from the donut pan. Then place on a wire rack to cool completely.
The topping is optional but so delicious and easy to make! To make the cinnamon sugar just mix the sugar and ground cinnamon in a small bowl until well combined.
Then use a brush to brush the melted butter onto the top of a donut and then immediately dip it into the bowl of cinnamon sugar.