In a small microwave safe bowl, microwave the butter until it’s melted.
Add the vanilla, milk and egg to the butter and whisk to combine.
Stir in the sugar, flour, baking powder, hot cocoa, and cocoa powder.
Divide the batter between two 8-ounce mugs filling no more than halfway in each mug. (Overfilling will result in a huge mess.) Sprinkle the batter with chopped truffle and marshmallows.
Place the mugs in the microwave and bake on high for 1 minute. The cake should spring back when pressed lightly with your finger. If the cake is undercooked, cook for an additional 15 seconds until cooked through.
