1. Preheat oven to 180°C Sift flour and cocoa into a large bowl. Stir in sugar and make a well in the centre.
2. Whisk egg, butter, and milk in a jug and pour into well. Stir gently until just combined (don't beat vigorously or the pudding won't be light).
3. Spread mixture into a 2L capacity ovenproof dish.
4. To make sauce, place boiling water in a glass jug. Add cocoa and sugar. Stir until dissolved. Pour evenly over pudding mix.
5. Bake for 40 minutes.
6. Microwave method:.
7. Microwave on high for 10 minutes. Test cake, If necessary, microwave a further 2 minutes.