Grab a small saucepan and pour in the fat-free milk. Gently sprinkle the gelatin over the top and let it hang out for about 5 minutes to bloom and soften.
Place the saucepan over low heat. Whisk continuously until the mixture becomes smooth and clear, with no more froth. Make sure not to let it boil, just warm it enough for the gelatin to fully dissolve.
Take the pan off the heat and let the mixture cool down for a few minutes. This prevents the protein powder from clumping.
Carefully transfer the cooled gelatin mixture into the bowl of a stand mixer. Now, add in your favorite protein powder.
Whip the mixture on high speed for about 5 to 8 minutes. Watch as it transforms into a fluffy, glossy, marshmallow-like cloud with soft peaks.
Line a baking pan or container with parchment paper. Pour the whipped mixture into the pan, spread it evenly, and pop it into the refrigerator for 2 to 3 hours, or until it's perfectly set.
Once firm, use a sharp knife to slice your protein marshmallows into bite-sized squares. For that classic touch, lightly dust them with powdered sugar (or a sugar-free alternative) and keep them stored in an airtight container in the fridge.