Make sure to use a medium-low to low heat. Don't let the onions burn, meaning the edges of them are turning black and they look actually burnt. Be especially when you first add them to the pan and they get black on the edges. This is bad and turn your heat way down! The caramelization process - even my sped-up version of it - is still just that: a process that takes some time. And it's best to err on the side of low-and-slow when it comes to how much heat is coming off your burner. Apart from them, you don't want to burn the butter!