I love lemon curd. So, I've decided I'm going to try my hand at making it. I've gone through my books and I've noticed that while most recipes call for the butter to be added off-heat after the egg/sugar/lemon juice mixture has thickened, one or two recipes tell you to throw all the ingredients in a double boiler and whisk until thickened. (I've narrowed it down to RLB's The Cake Bible and Sherry Yard's The Secrets of Baking specifically.) Does it make a difference when the butter is a