Preheat the oven to 325°F (165°C) and line a baking sheet with parchment paper.
Add the egg whites and cream of tartar to a completely clean mixing bowl.
Beat until the egg whites form stiff peaks.
In a separate bowl, mix the sugar-free cake mix and protein powder.
Add the dry mixture to the whipped egg whites a little at a time, gently folding after each addition. Avoid stirring or beating the mixture.
Divide the batter into 5 large mounds on the prepared baking sheet.
Sprinkle the tops with the sugar-free sprinkles.
Bake for 16–18 minutes, until the cookies are puffed and lightly golden.
Let them cool completely on the baking sheet. They will firm up more as they cool.
