Bread-and-butter letter

What is Bread-and-butter letter?

Bread-and-butter letter definition and meaning on Dictionary terms:
a kind of food made of flour or meal that has been mixed with milk or water, made into a dough or batter, with or without yeast or other leavening agent, and baked.
food or sustenance; livelihood: to earn one’s bread.
Slang. money.
Ecclesiastical. the wafer or bread used in a Eucharistic service.

verb (used with object)
Cookery. to cover with breadcrumbs or meal.