In a heatproof mug or ramekin, stir together the dry ingredients until no lumps remain. Stir in the oil and milk until you have a thick paste. Stir in the chopped chocolate or nuts, if you’re using them.

Microwave on high for a minute, checking it after 30 seconds (microwaves vary) – it will be done when it’s springy on top but still a bit gooey – like the very best brownie. Eat warm.