Also, *white* chocolate is even more tempermental when melting. The hotter
the ingredient gets, the more solid it gets...until it crumbles.
(written from valid experience).
If using a microwave, use a lower power rating, use 30 second limits, stirring in-between to distribute the heat and finally, use a HEAVY microwaveable plastic (like old Tupperware). Glass or thin plastic will transfer too much heat. Again, spoken from experience.