Preheat your oven to 180°C (350°F) and lightly spray a donut tin with non-stick spray.
In a large bowl, whisk together the flour, protein powder, sugar, baking powder, salt, and cinnamon.
Pour in the almond milk, applesauce, melted butter, and vanilla extract. Mix well with a wooden spoon until combined. The batter should resemble a cake batter consistency—neither too thick nor too runny. Adjust with a little more milk if needed.
Spoon or pipe the batter into the prepared donut tin, filling each cavity about 3/4 full. Be careful not to overfill as the donuts will rise during baking.
Bake in the preheated oven for 10-15 minutes or until the edges are lightly golden and a toothpick inserted comes out clean.
Allow the donuts to cool for a few minutes in the pan, then carefully transfer them to a wire rack to cool completely.
In a small bowl, combine the sugar and cinnamon for the coating. Lightly brush or dip each donut in the melted butter, then roll them in the cinnamon sugar mixture.
Enjoy these delicious cinnamon sugar donuts warm or at room temperature!