How do I melt white chocolate without it turning lumpy? And is there anything I can add to it to salvage it?
White chocolate is different to plain (bittersweet) and milk chocolate as it contains no cocoa solids but is usually made with cocoa butter. It can easily "sieze" and turn lumpy or grainy when melting and also scorches very easily. Generally it is much better to melt white chocolate in a bowl over a saucepan of warm water, rather than in a microwave, as it is easier to keep control over the temperature. Choose a bowl that sits snugly over your saucepan. The bowl should not come into contact with the water in the saucepan as this can cause over heating and the water only needs to be at the barest simmer (all chocolate melts at just above body temperature and white chocolate scorches at around 44c/110F). Chop your chocolate into even-sized pieces, or use chips, to help the chocolate to melt evenly.
Melted white chocolate will tend seize on contact with small amounts of liquid so any flavourings should be added to the bowl with the chocolate before it is melted, and stirred in as the chocolate melts. Also make sure that no steam from the water comes into contact with the chocolate, that the spoon you are stirring with is completely dry and do not cover the bowl as this will cause condensation which will drip into the chocolate. It is also a good idea to remove the bowl from the saucepan while there are still a couple of small lumps of unmelted chocolate in the bowl, they will generally melt if you continue to stir the chocolate and this way it reduces the risk of your chocolate over heating. If they don't melt just put the bowl back over the saucepan of hot water for a minute or two. Seized chocolate can occasionally be rescued by whisking in a small pat of butter. If this doesn't work then the chocolate can be used in other recipes where melted chocolate is added to a batter (such as cakes or cookies), but won't be smooth enough for coating.