Finely grate 40g carrot and add it to a microwave-safe mug with the egg and Greek yogurt. Mix thoroughly with a fork until the egg is fully incorporated.
Add the almond milk, zero-calorie sweetener, and vanilla extract, then mix again.
Add the oat flour, vanilla whey protein powder, cinnamon, ginger, nutmeg, baking powder, and salt. Stir until you have a thick, evenly mixed batter with no dry pockets.
If the batter is unusually thick, add another teaspoon of almond milk and mix again.
Microwave on high for about 75 seconds. Check the center and continue in 10–15 second intervals if necessary. In a typical 1,000-watt microwave, the total cooking time will usually be about 75–90 seconds.
Remove the mug carefully and let the cake rest for about 1 minute. The center should be set but still soft and moist rather than completely firm.
Serve warm as written or top with a spoonful of Greek yogurt, cinnamon, berries, chopped walnuts, or another topping that fits your nutritional goals.