Preheat and prep: Heat oven to 350°F (175°C). Grease your donut pan well, especially the center posts.
Mix dry ingredients: In a large bowl, whisk oat flour, protein powder, baking powder, baking soda, salt, and cinnamon until evenly combined.
Mix wet ingredients: In another bowl, whisk eggs, almond milk, Greek yogurt, sweetener, vanilla, and melted oil until smooth.
Combine batter: Pour wet into dry and stir gently until just combined. The batter should be thick but spoonable.
If it’s too thick, add 1–2 tablespoons of milk. If it’s runny, sprinkle in a little more oat flour.
Fill the pan: Spoon or pipe batter into each cavity, filling about 3/4 full. Smooth the tops with a spoon for even donuts.
Bake: Bake 10–14 minutes, until the tops spring back lightly and a toothpick comes out clean.
Avoid overbaking; protein treats can dry out fast.
Cool: Let donuts rest in the pan for 5 minutes, then transfer to a wire rack. Cool completely before glazing.
Glaze (optional): Whisk powdered sweetener, vanilla, and just enough milk to make a pourable glaze. Dip tops of cooled donuts, let excess drip off, and set on a rack.
Add toppings while the glaze is wet.