A familiar scenario:
You pick up your latest issue of
Taste of Home/Bon Appetit/Rachel Ray magazine and actually take the time to cut out that amazing white chocolate cheesecake recipe. You make a list of ingredients, as you vow that this time, no matter how rare or expensive, you will purchase every one in the recipe. You actually remember to bring the list to the store, and you bring everything home with you.
You prep your recipe. It calls for 8-ounces of melted white chocolate. So you put it in the microwave and wait. But something happens...as it melts...it hardens. So you think, "I'll heat it up again," but it becomes even more unmanageable. Two expensive baking bars trashed. Your recipe ruined. You cancel your subscription and drown your sorrows in the brick of cream cheese and granulated sugar you never got to use.
There are a few things at play here. First, the darker the chocolate, the higher the cacao content, the easier it is to melt. Dark chocolate is a dream to melt, will rarely burn, and will hold its consistency for a long time. Milk chocolate is OK, and white chocolate is simply a nightmare. White chocolate is only allowed to use the word "chocolate" because it contains cocoa butter. No cacao required. Cocoa butter shouldn't be a problem to melt, except that there's probably more in that bar of white chocolate than you anticipated. If cocoa butter isn't the first ingredient, then you don't stand a chance of melting it effectively. Next, if any moisture gets into your chocolate, like from your double-boiler should you choose that method, or condensation within the microwave, the chocolate seizes up immediately. This is true if you add water to any kind of chocolate, but white chocolate is particularly sensitive.
The best advice is to melt the chocolate at 50% power in your microwave at 20 second intervals, stirring in between. This will prevent it from burning. Once the chocolate has lost its form, stop melting it. You won't get it to be as smooth as dark chocolate, and for most recipes, just getting the chocolate soft is enough to mix it thoroughly into your recipe. White chocolate sold at your local supermarket will pose this challenge more frequently than high-end white chocolate the professionals use. High quality white chocolate (AKA: expensive) will be easier to work with because it will contain fewer ingredients (like vegetable fats and oils - cheap substitutes) to get in the way. But most grocery stores will only give you a selection of 2-3 brands to choose from, making it difficult to find the best chocolate to use in your recipe.
If you are using the white chocolate for dipping or coating, then adding a little bit of butter to the mixture will help smooth it out. Think 1 tbsp. for every 3-4 ounces of chocolate. Of course, the irony is that you will be adding more ingredients - but sometimes that's the only way to get the white chocolate to work with you.
If all of this still frustrates you, then try making a white-chocolate-CHIP-cheesecake instead. Enjoy!