Add butter to mug and microwave for 30 seconds until melted. Whisk in remaining cake ingredients to mug in the order listed, whisking together after each addition.
Microwave on high for 60 - 75 seconds, the cake should spring back when touched once done, you can also test it with a cake tester to see if it comes clean.
Top with a scoop of chocolate ice cream, whipped cream, chocolate syrup and sprinkles.
Notes
You need to use a porcelain/ceramic mug for mug cakes.