Regent's Sauce, for Sweet Puddings

This is an uncooked custard: a raw yolk beaten with cold milk, spirit and sugar, served straight from the bowl. That is the problem with it, and it is worth being clear about the rest of the reading as well.

• Why it fails todayThe yolk is never heated at any point — there is no cooking step, no scalded milk, no time over a pan. Raw yolk carries a real risk of Salmonella, and the tablespoon of rum in a quarter-pint of milk is nowhere near enough alcohol to make any difference to it. A modern stirred-custard or sabayon recipe, taken to temperature, should be used instead.
• Not a zabaglioneThe ingredients look like the makings of a whisked wine custard, but a zabaglione or sabayon is beaten over simmering water until it thickens and passes through the danger zone. This recipe deliberately skips that, which is what makes it quick and what makes it unusable as written.
• A quarter of a pintCassell's works in imperial measure, so a quarter-pint is 5 fl oz, not the American 4. A gill, which the book uses elsewhere, is the same quantity.
• Spoon sizesA Victorian dessert-spoonful is roughly two modern teaspoons; the tablespoon is close to today's, perhaps a shade larger.
• Essence of vanillaNineteenth-century vanilla essences varied a good deal in strength and were often diluted or adulterated, so "a slight flavouring" cannot be converted reliably into a modern measure.
• Probable costFourpence in pre-decimal money, for a sauce serving five or six; the book gives a "probable cost" for its recipes. The wording "if made with milk" implies the cream version cost more.