Combine the warm water and active dry yeast in a small bowl and let it sit for 5 minutes until it becomes foamy.
In a large mixing bowl, whisk together the oat flour, vanilla whey protein powder, sea salt, and ground cinnamon.
Add the non-fat Greek yogurt, egg, vanilla extract, and the yeast mixture to the dry ingredients.
Stir the dough until well combined, then cover the bowl and let it rest in a warm spot for 20 minutes to allow the yeast to activate.
Preheat your oven to 350 degrees Fahrenheit and lightly grease a donut pan with a small amount of coconut oil.
Transfer the dough into a piping bag or a plastic bag with the corner cut off and pipe the mixture evenly into the donut pan.
Bake for 12 to 15 minutes until the donuts are firm to the touch and a light golden color appears.
Remove the donuts from the pan and allow them to cool slightly on a wire rack.
Drizzle the maple syrup over the warm donuts as a glaze before serving.