Preheat oven to 350 degrees and lightly grease a donut pan with non-stick spray. Set aside.
In a medium bowl combine flour, protein powder, cocoa powder, baking powder, and salt. Set aside.
In a large bowl or stand mixer combine egg, sugar, milk, butter and vanilla.
Slowly add in the flour mixture and mix until well combined.
Fill your prepared donut pan with your donut batter. This should make ~6 donuts.
Bake for 12 minutes or until a toothpick when inserted comes out clean.
Allow donuts to cool fully before frosting.
To make the frosting combine all frosting ingredients in a small bowl. Your frosting should be runny enough to dip your donuts into it for a nice glaze. If it is not, pop it in the microwave to melt the butter a bit and it should get runniner. If it does not then you can add ¼ tsp of milk at a time until you reach the desired consistency (see video in folder to see how it should look!)
Dip your donuts top side down (The side facing UP in the donut pan faces DOWN in the frosting) then sprinkle with rainbow sprinkles. Repeat with remaining donuts. As the butter cools your frosting will begin to harden a bit! This is good.
Enjoy!!!