The recipe comes from 500 Delicious Dishes from Leftovers. Mr. Big Food is almost finished scouring this
crappy old cookbook looking for new additions to The Big Food Manual. Volume 3 is cakes!
The recipe in Big Food begins,
“Sprinkle with sugar and cinnamon and serve with roast duck or turkey.”—The Encyclopedia of Cooking Complete in 24 Volumes, Volume 2: 500 Delicious Dishes from Leftovers (1953)
Sift together flour, baking powder, salt, sugar, and nutmeg. Beat together egg yolk and milk, add dry ingredients, and bat until mixture is smooth. Add sweet potato and fat, and continue beating. Fold in stiffly beaten egg white. Bake on a heated greased waffle iron until brown.