A double boiler is actually going to apply more gentle heat than simmering. Butter is going to have a tendency to burn so you'd want to use a double boiler or crock pot--something that is going to apply indirect heat. Simmering is okay for cooking oil as it doesn't burn at such low temperatures.

Give me organic butter any day of the week over "processed easily spreadable butter flavored goo" that comes in the hermetically sealed industrial size tubs. You want to talk chemicals?