Preheat your oven to 350°F and lightly grease a standard six-cavity donut pan with a touch of oil or ghee.
In a large mixing bowl, whisk together the Greek yogurt, egg whites, maple syrup, and vanilla extract until the mixture is completely smooth and well combined.
Add the oat flour, baking powder, one teaspoon of ground cinnamon, and sea salt to the wet ingredients, stirring until just incorporated into a thick batter.
Carefully spoon the batter into the prepared donut pan, filling each cavity about three-quarters of the way to allow for rising.
Bake for 15 to 18 minutes, or until the donuts are springy to the touch and a toothpick inserted into the center comes out clean.
While the donuts cool for a few minutes, melt the ghee in a small bowl and combine the monk fruit sweetener with the remaining half teaspoon of cinnamon in another small bowl.
Brush the tops of the warm donuts lightly with the melted ghee and immediately press them into the cinnamon-sugar mixture to coat thoroughly before serving.