Melt the butter in a 12-ounce (or bigger) microwavable mug or ramekin. Swirl the butter around to coat the mug or ramekin. Add the milk, vanilla, maraschino cherry juice and egg and whisk. Add the sugar, cocoa powder, flour and baking powder and whisk.
Microwave for 90 seconds or until the cake has risen and is set. Let cool slightly, top with whipped cream and a cherry.