White chocolate should contain a minimum of 20% cocoa butter (this is an EU standard). Generally speaking, the higher the percentage of cocoa butter, the better, so aim for higher than this level if you can (it's possible to find bars with up to 36% cocoa butter). Avoid any bar with less than 20% cocoa butter as it most likely contains a high proportion of vegetable oil. The chocolate should also contain milk powder, sugar and emulsifier plus any natural flavourings (such as vanilla). Bars listing additional ingredients, such as vegetable oil or palm oil, are causes for concern and are best avoided.