1. Place the butter in a bowl and microwave it for 30 seconds until melted.
2. Add the egg and whisk with a fork.
3. Stir in the milk, vanilla, and sugar.
4. Add the flour and mixed spice. Mix the batter with a fork until smooth.
5. Pour the batter into a mug. Microwave for 1 minute until risen. Take care not to overcook.
6. Decorate with some sieved icing sugar and top with a dollop of brandy butter. Serve immediately.