How did the term 'Buttered the bread' originate?
Back when actual butter was the main spread to put on bread, it took a very gentle touch to spread butter straight from the refrigerator (icebox) without tearing up the bread.  Thus, a very smooth landing might be said to have "buttered the bread" because it went on smoothly enough to keep from tearing anything up.

This probably also evolved into "greased it on" or "greaser" to refer to a similarly smooth landing.