In a microwave-safe mug (minimum size 1 cup/250ml), whisk together egg, pumpkin puree, maple syrup, nut butter, cinnamon and nutmeg. Stir until you get a smooth pumpkin mixture.
Sift in flour, protein powder and baking powder. Stir to combine, but do not over-mix the protein batter.
Microwave on high power for about 60 seconds, or until you see the protein mug cake rising. Remove from the microwave carefully and check the texture. It should be moist and soft.
Serve warm with a scoop of vanilla ice cream and a sprinkle of cinnamon, if you desire.