Add the protein powder, baking powder, and sweetener to a large microwave-safe mug. Use a small whisk or fork to break up any clumps and make sure all the dry ingredients are evenly combined.
Add the Greek yogurt, egg white, almond milk, and vanilla extract directly into the mug. Whisk thoroughly until the mixture becomes smooth and no dry pockets remain. The batter should look like thick pancake batter — if it feels too thick, add 1–2 teaspoons of almond milk and mix again.
Place the mug in the microwave and cook on high for 60–90 seconds. Start checking at the 60-second mark. The cake should rise and look set on top, but still moist. Avoid overcooking, as it can turn rubbery.
Remove the mug carefully — it will be hot. Let the cake rest for about 1 minute so it can finish steaming and settle. Enjoy warm as is, or top with Greek yogurt, berries, or sugar-free syrup.