milk or cream, wine or ale, egg, bread, grain, nuts, and/or spices
Posset, and the comparable syllabub, are medieval concoctions that are perhaps today easier to relate to from a culinary perspective than a drinks perspective: they involve curdling milk or cream in an acidic wine or ale, yielding an edible layer of proteinous curds with a drinkable whey-infused mixture below. Possets (warm) and syllabubs (cool) remained popular for centuries, largely disappearing by 1800, but they comprise an essential link in the history of dairy in mixology. (See also milk punch, egg nogg, and flip.)
Shown above is one posset recipe of a great many. In this case, an egg posset from Oxford Night Caps (1827). We selected it as an example because it happens to tie in nicely with later egg drinks.
Oxford Companion to Spirits and Cocktails, p. 553–4, 706–7; barware icons courtesy of Haus Alpenz