In a small bowl, beat the egg. Set aside. In a separate, slightly larger bowl, mix cocoa powder, Truvia, and salt. Add milk, vanilla, and egg to the dry ingredients. Mix well. Add baking powder and mix again.
3
Firmly tap the mug on the counter to release any air bubbles.
4
Cook for roughly 1 minute 20 seconds, depending on your microwave. The top of the cake should be solid. Flip onto a plate to cool.