In a bowl, beat together the oil, egg, sugar and flour until smooth. Pour roughly half the mixture into another bowl.
To one of the bowl add the orange zest and juice and mix together until combined.
Tot the other bowl add the cocoa, heavy cream and salt and stir thoroughly.
Into a microwave proof mug drop alternate tablespoons of the orange mixture and the chocolate mixture until you have used up both.
Microwave for 1 minute 30 seconds at 800W. The cake will rise and still be quite we to the touch.
Leave the mug cake to cool for 1 minute before eating. Garnish with a bit of orange zest or chocolate shavings for serving.
Notes
My microwave is 700W so I cooked it a bit longer. If yours is more than 800W adjust cooking time. Most microwaves have the wattage on the door or interior of the unit.
If making more than one mug cake, cook each separately. They don't take that long!