According to The Cocktail Key, a little pamphlet brought into this world by Herbert Jenkins Ltd, 3, Duke of York Street, London, S.W.1 (that’s the address given on the cover), the ingredients of a port flip are as follows.
1) a glass of port. (I told you this wasn’t a trick question.)
2) a dash of sugar syrup (as though port wasn’t sweet enough already).
3) the yolk of one egg.