Not Lemon Butter. It's Lemon Curd.

One of my pet hates is the misnomer ‘lemon butter’. In fact, there is no butter in fruit butters! The term refers to the consistency of fruit that has been sieved or pureed so the resulting spread is like that of room temperature butter. The proper name is ‘lemon curd’. The following quote (and the recipe below) from “The WI [Women’s Institute] Book of Jams and Other Preserves” explains the differences between butters, cheeses and curds.