Heat oven to 170C/fan 150C/gas 5. Brush 6 holes of a muffin tin with a drop of oil. Sieve the cocoa into a large bowl, then add the rest of the dry ingredients. Stir to combine, then make a well in the centre.
Beat the milk, egg and oil together in a jug, pour into the well, then stir quickly to make a batter. Spoon into the muffin tin, then bake for 15 mins or until risen and firm to the touch. Heat the custard in the microwave according to instructions, or in a pan, then tip in the chopped chocolate and stir until smooth. Turn the puddings into bowls and pour the custard over.