Boiling wine with spices is a tradition throughout Europe and the world. This is a variant using apple wine from Frankfurt.
This dish, similar to French Toast, dates back to the 16th century, perhaps even earlier. According to legend, it comes from the order of Carthusian Monks who practiced a strict vow of poverty and community service. It uses only ingredients that would have been easily accessible to anyone at the time, and the recipe requires no special knowledge or skills to prepare. While the Carthusian monks originated first in France, this dish appears to be from somewhere in Catholic Southern Germany, likely Swabia or Franconia.
This recipe dates to the 17th century Medici courts in Florence but was a variation of a common drink at the time. Originally, eggs were used to thicken common drinks like beer or wine, but later the dish was used to fill pastries and other desserts. This is a very simple version, and it can be changed in various ways, notable by adding spices or using different wines.
This recipe is for a simple cream sauce to go with any dessert. It's technically not custard, but it's close enough. It takes advantage of the Frankfurt specialty drink apple wine but could be made with any dry white wine. Using apple cider may require some adjustment to the sugar amount.
Himmel und Erde or Heaven and Earth refers to the two kinds of potatoes in the dish, the sky apples, i.e., apples from trees, and earth apples, i.e., potatoes. It is essentially mashed potatoes with blood sausages, either as a puree or fried. A common dish from the Rhineland in Germany, it was once a staple of working class households.