Butter is a co-dominant colour and pattern mutation. It is also allelic to Lesser Platinum, which means the gene sits in the same place on the DNA strand.
Butter and Lesser Platinum are both part of the Blue Eyed Leucistic complex, along with Mojave (among others). Any combination of these (Butter + Butter, Butter + Lesser, Butter + Mojave, Lesser + Mojave etc) would have the chance of producing a Blue Eyed Leucistic.
Butter and Lesser are so similar that it is possible that they are the same mutation discovered by two different people who each gave it a different name.
We are working with both mutations. We treat them as separate morphs and call them by the name we bought them as.