Sift flour, cocoa powder and salt into a small bowl, stir through the sugar.
In a small separate bowl, whisk the egg, milk, butter and vanilla. Add the wet ingredients to the dry ingredients and stir with a fork until just combined.
Spoon mixture into a 2 cups capacity (500ml) mug, or two smaller tea cups.
Microwave on high for 70-90 seconds, or until pudding has risen and top is just set. Do not overcook. Dust with extra cocoa powder, serve immediately with a little pouring cream.