According to Andrew Smith, editor of the Oxford Encyclopedia of Food and Drink in America, explains that a “sugar sandwich” is a generic sandwich with sugar added, such as a banana sandwich with sugar sprinkled on the bananas, or homemade peanut butter, which is not particularly sweet, with sugar.
Lucy Long, a folklorist, says that Sugar Bread was a common snack in the American South made with bread (white was preferred) spread with butter and sprinkled with white sugar.
Jenny Ravens, Sidney, Australia, remembers her grandmother occasionally sprinkled sugar on bread and butter as a treat for the grandchildren. We could then choose “church windows” (diagonal crosshatches) or “house windows” (square crosshatches) as decoration. Jelly crystals on bread and butter were common at children’s parties, though they were usually closed sandwiches.
Fabio Parasecoli, a food historian based in Rome, ate sugar sandwiches as a snack in Italy. Parasecoli’s recipe calls for slices of hard bread softened with water, spread with butter, and sprinkled with sugar.
See Conrad Aiken’s “Strange Moonlight” (1925)