Add the butter to a large mug. Micwoarve until melted. Add the remaining ingredients to the mug. Stir with a fork until mixed through and lump-free. Drop-in a few sugar-free chocolate chips or chunks if desired.
Microwave for 1 minute. Cool for 3-5 minutes before serving. Serve with whipped cream, keto ice-cream, or your favorite toppings!
Total Net carbs 5g is calculated by subtracting fiber and sugar alcohol from the erythritol.