Mix cocoa and butter and add rest of ingredients except baking powder. (“Vinegar” is an error, perhaps someone was trying out the cooking sherry while transcribing this)

Beat 3 minutes then add baking powder.

Bake for 20-35 minutes (a broad range, just keep checking it so it doesn’t go underbaked or dry out completely) at 190°C. Use a greased, lined shallow square tin and you could turn this sponge into a swiss roll! The instructions don’t go into bake time x tin size/shape, so definitely one to watch closely until you’ve got the hang of how it behaves in your tin, and in your oven.)