In a microwave-safe mug, add butter and heat in microwave until completely melted. Only heat a few seconds at a time, swirling in between, so that the butter does not overheat and splatter.
Once butter is melted, add in pumpkin puree and then egg. Whisk vigorously until batter is smooth and no egg streaks remain.
Add in remaining ingredients. Whisk until batter is smooth.
Cook cake at full power in the microwave for about 1 minute 45 seconds to 2 minutes, or until cake is cooked. Let cake cool a few minutes before adding toppings of your choice and serving.
