Some types of white chocolate have had some or all of the cocoa butter replaced with vegetable oil and these are extremely difficult to melt. White chocolate should never be directly substituted for dark or milk chocolate. Technically, white chocolate is not chocolate at all as it does not contain any cocoa solids. Should a particular recipe require white chocolate and it has to be melted, check that the type you buy contains a high level of cocoa butter and always chill and then finely grate before very gently melting it. If it seizes, white chocolate is impossible to rescue.
** Asian Recipes