Place the butter in a microwave-safe mug and melt by heating in the microwave for 30-45 seconds. If the butter is hot, allow it to cool before mixing it with the egg yolk.
Add the egg yolk, almond milk, and extract to the melted butter and stir well.
Mix in the Keto Chow, coconut flour, allulose, and baking powder, then stir in the sprinkles.
Microwave on full power for approximately one minute, until the cake is firm. It may look slightly uncooked on the top. Careful: the mug may be very hot. Allow the cake to cool.
While it cools, make the frosting. Mix the melted butter, heavy cream, allulose, and Keto Chow, until smooth.
Serve in the mug or turn out onto a small plate, frost, and decorate if desired.
Notes
Use a large mug so the cake has room to rise. No need to oil the mug; the cake should pull away from the sides as it bakes.