This section is from the book "The American Woman's Cook Book", by Ruth Berolzheimer. Also available from Amazon: The Domestic Arts Edition of the American Woman's Cook Book.
Preparing the Butter - The butter should be thoroughly creamed before it is used or it will not spread evenly over the bread. To cream butter, place it in a warm bowl and mash and beat it until it is soft. It will then spread well even on fresh bread. Sandwich butters are often made by creaming one cup of butter with one-half cup of cream. One-half cup of butter, creamed, will spread a two-pound sandwich loaf cutting forty to forty-five slices.
Spreading Butter and Filling - A poorly buttered sandwich is very unpalatable. Spread the butter to the very edges of the slices, on the sides that are to be put together, being careful, however, not to let the butter spread over the edges So that it is untidy. If the slices need not be fitted together, it is often easier to spread the bread before cutting it from the loaf. A pliable knife or small spatula is a help in spreading butter or filling.
Spread the filling on the buttered surface of one slice only of each sandwich. Have the filling come to the edge of the sandwich, if possible.
When mayonnaise is used, not combined with a filling, as in mayonnaise and lettuce sandwiches, it is more evenly distributed if it is spread on one of the slices of bread and the lettuce leaf placed upon it.