Preheat your oven to 180°C (350°F) so it’s fully heated before baking—this helps the doughnuts rise properly and develop a soft, fluffy texture. Lightly grease your mini doughnut pan with a small amount of oil or non-stick spray, making sure to coat each cavity evenly so the doughnuts release easily after baking.

In a medium mixing bowl, add the Greek yogurt, egg whites, milk, and vanilla extract. Whisk everything together until the mixture becomes smooth, creamy, and fully combined with no lumps. This step ensures the batter has an even texture and helps the doughnuts turn out soft rather than dense.

Add the oat flour, protein powder, sweetener, and baking powder into the wet mixture. Using a spatula or spoon, gently fold everything together until a smooth batter forms. Be careful not to overmix—stop as soon as everything is combined, as overmixing can make the doughnuts dense instead of light and airy.

Transfer the batter into a piping bag or a ziplock bag with one corner cut off. Carefully pipe the batter into each doughnut mold, filling them about three-quarters full. This gives the doughnuts enough space to rise without overflowing and helps them maintain a neat, round shape.

Place the filled doughnut pan into the preheated oven and bake for 10 to 12 minutes. Keep an eye on them toward the end—the doughnuts are ready when they look lightly golden on top and gently spring back when you press them with your finger. Avoid overbaking, as this can dry them out.

Remove the pan from the oven and allow the doughnuts to cool in the pan for about 3–5 minutes. This short resting time helps them firm up slightly, making them easier to remove without breaking.

Carefully take the doughnuts out of the pan and place them on a rack or plate. Serve them warm for the best soft and fluffy texture, or let them cool completely if you plan to store them for later.