Mix Happy Mug cake according to the packaging procedure. Set aside.
Mix butter, sugar and cinnamon powder in a bowl. Mix well.
Transfer butter mixture in a piping bag and cut the tip of the piping bag and insert it in the middle of the happy mug cake slowly swirling starting at the center and slowly pulling it up, up to the top of the mug cake batter.
Microwave the happy mug cake for 1 minute and 30 seconds.