Melting White Chocolate Chips

I have a recipe for peppermint brownies that involves pouring on a semisweet chocolate glaze, then drizzling with melted white chocolate - which is then feathered by dragging a toothpick through it. I have no problem with the concept, but I've had trouble melting the Ghiradelli white chocolate chips I've been trying to use.

The directions for melting the chips are to put them in a double boiler, set over gently simmering water, making sure the bottom of the insert isn't touching the water, and stir frequently until the chips melt. Each time I try this (and I've tried several times), the chocolate seems to start melting, then seizes into a huge clump. I tried reducing the heat under the pot and eventually got the white chocolate to soften up again, but never have been able to get it to the point where I could actually *drizzle* it over the chocolate glaze.

Can anyone tell me what I'm doing wrong?