Chocolate Custard (Scrape half a cake of good)

A cross-reference recipe: it assumes you already know how to make a boiled custard and only tells you how to get the chocolate into it. The gaps are the yolk count, the sugar and the finishing temperature.

• "Half a cake of chocolate"Chocolate was sold in pressed cakes or blocks, but sizes differed from maker to maker and the recipe gives no weight, so this is genuinely ambiguous. Around 2 oz of unsweetened or dark chocolate to a pint of dairy gives a custard in keeping with the period.
• Sugar is not listedSome period cakes of "good chocolate" were lightly sweetened and spiced, others not, and the recipe simply does not say. Count on 3 to 4 tablespoons of sugar per pint if using unsweetened chocolate, dissolved with the milk.
• YolksA stirred custard on a pint of milk and cream wants 4 to 6 yolks; four sets softly for a pouring sauce, six gives a spoonable cream. "Milk and cream" here reads as a mixture rather than one or the other; half and half of each is a fair reading.
• Moisten by degreesThis is the hinge. Scraped chocolate flooded with hot liquid seizes into grit; add a few spoonfuls of warm milk first and work it to a smooth paste, then let the rest down gradually.
• Tempering the yolksBeat the yolks, then pour the warm chocolate milk into them in a thin stream while whisking before returning the lot to the pan. Adding yolks straight to a hot pan gives you chocolate scrambled eggs.
• Finishing temperatureCook over low heat, stirring, until it coats the back of a spoon — 175 to 180F. Above about 185F the yolks curdle. A pan set in simmering water is the safe way if the stove runs hot.
• StrainingPass the finished custard through a fine sieve into a cold bowl. It catches any set egg and any unmelted chocolate, and stopping the residual heat keeps it from tightening as it stands.
• YieldA pint of dairy makes roughly four small custard cups, or enough sauce for six servings of pudding. Chill covered; a skin forms otherwise.