Pour the fat-free Fairlife milk into a small saucepan. Sprinkle the unflavored gelatin evenly over the milk. Let it sit for about 5 minutes without stirring; this allows the gelatin to "bloom" and absorb the liquid, ensuring a smooth mixture later.
Place the saucepan over low heat. Continuously whisk the mixture until the gelatin completely dissolves and the liquid thins out, becoming smooth and no longer frothy. It's important not to let it boil, just warm it enough. Once smooth, remove the saucepan from the heat and let it cool for a few minutes. This cooling step prevents the protein powder from clumping.
Carefully transfer the cooled milk and gelatin mixture into the bowl of a stand mixer (or use a large bowl with a hand mixer). Add your chosen whey protein powder.
Whip the mixture on high speed for about 5 to 8 minutes. Watch as it transforms into a light, airy, and glossy cloud-like consistency, forming soft peaks. This is where the magic happens!
Carefully pour the fluffy marshmallow mixture into a baking pan or container that's been lined with parchment paper. Smooth the top evenly with a spatula. Pop it into the refrigerator for 2 to 3 hours, or until it's completely set and firm.
Once firm, gently lift the marshmallow block out of the pan using the parchment paper. Use a sharp knife to slice it into individual squares. If you like, lightly toss the marshmallow squares in a dusting of powdered sugar or a sugar-free alternative for a classic finish.