Preheat oven to 325°F (160°C). Line a baking sheet with parchment paper.
In a large bowl, combine oats, frosted flakes, protein powder, cinnamon, and salt.
In a separate bowl, whisk egg whites until frothy (about 30 seconds). Stir in vanilla extract if using.
Pour egg whites over dry mix and stir until everything is lightly coated. Mixture should be damp but not wet.
Spread mixture evenly on prepared baking sheet, pressing lightly to form clusters.
Bake for 20 minutes, stir, then bake an additional 10–15 minutes until golden and crisp.
Cool completely before storing (it crisps up more as it cools).
