Melt the butter in one large (12-ounce or bigger) or two small (8-ounce) microwavable mug(s) or ramekin(s). Swirl the butter around to coat the mug or ramekin. Add the milk, vanilla, and eggs and whisk. Add the cocoa powder, flour, baking powder, and maple syrup and whisk.
Microwave on high power for 90 seconds for small ramekins, about 2 minutes for large or until the cake has risen and is set. Let cool slightly, top with whipped cream or coconut cream if desired, and enjoy!