Whisk together flour, sugar, Sleep Tight Hot Choccy, baking powder, and a pinch of salt in a small bowl.
Add in hot water, melted butter, and vanilla. Stir together well ensuring there are no lumps.
Pour into a large microwave-safe mug and sprinkle chocolate chips over the top of the batter.
Heat on high for 1 minute then use a toothpick to check if it's cooked inside (if it comes out mostly clean your mug cake is ready!). If it's still too moist on the inside, heat for an additional 15 seconds, then check again—a few crumbs or bits of chocolate chips are ok. Repeat until your mug cake is cooked through.
Allow your cake to cool for a few minutes then add your favourite toppings and enjoy while it's still warm. We used whipped cream, a dusting of cacao and marshmallows!