Put the butter in a large heatproof mug and melt in the microwave on high for 30 seconds. Add the milk, egg, sugar and vanilla, and mix well.

Stir in the flour, cocoa powder, baking powder and a pinch of salt using a fork until smooth. Push the square of chocolate down into the middle of the mixture.

Cook in the centre of the microwave on high for 1 1/2-2 mins, or until risen and firm to touch. Top with whipped cream, marshmallows and chocolate shavings, and serve warm.