Place the xylitol, flour, cocoa powder and baking powder into a 300ml mug
Mix together using a fork then add the oil, milk, vanilla extract, and egg
Mix until just smooth then place one chocolate block on top of the batter
Place into a 900W microwave and microwave on 100% power for 1 minute and 10 seconds
Remove from the microwave and place the other chocolate block on top of the cake and as soon as it starts to melt, sprinkle with some flaked salt and dive in