You're spoilt for choice, as sons called William were born in 1756 and 1763 to William Cornfoot and Henrietta Anderson in Kilconquhar; in 1761 to William Cornfoot and Margaret Briddie [= Brydie?] in St. Monans, and in 1764 and 1766 to Walter Cornfoot and Sophia Smith in Kilconquhar.
If you know the names of all the children born to Wm. Cornfoot and Margaret Brown, that should help pinpoint the names of his parents.
Do you know the story of Janet Cornfoot, the Pittenweem witch?
Harry