Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper or lightly grease a mini muffin tin.
In a mixing bowl, whisk together the oat flour, protein powder, baking powder, baking soda, salt, and cinnamon.
In another bowl, whisk the Greek yogurt, milk, egg, sweetener, vanilla, and light butter or oil until smooth.
Pour the wet ingredients into the dry.
Stir gently until just combined. The batter should be thick but scoopable. If it’s too dry or crumbly, add 1–2 tablespoons more milk.
Use a small cookie scoop or spoon to portion 1–1.5 tablespoon mounds.
For a baking sheet, roll lightly with damp hands into balls. For a mini muffin tin, simply scoop into each cavity.
Bake for 9–12 minutes, until the tops spring back when touched and a toothpick comes out clean. Avoid overbaking so they stay moist.
Cool for 5 minutes.
For the cinnamon “sugar,” brush or spritz each donut hole with a tiny bit of melted butter or oil, then roll in the cinnamon-sweetener mix. For the glaze, whisk the ingredients until smooth and dip the tops, letting excess drip off.
Let set for a few minutes, then enjoy warm or at room temperature.