Pre-heat oven to 375 degrees. Spray a donut pan with non-stick cooking spray and set aside.
Combine all dry donut ingredients in a bowl. Add almond milk, Greek yogurt, egg, apple cider vinegar, maple syrup and vanilla extract and mix until smooth. Add melted peanut butter and mix until smooth again.
Spoon donut batter into donut pan evenly.
Bake at 375 degrees for 10-12 minutes or until a toothpick inserted into a donut comes out mostly clean. Remove from donut pan immediately and flip onto a cooling rack.
Whisk together casein protein powder and almond milk until smooth, then add cream cheese and continue to whisk until smooth.
Once donuts have cooled, dip into frosting or spread frosting on top of donuts. Top with sprinkles if desired.