Taken from the Heal My Hands blog at http://healmyhands.typepad.com/heal_my_life/2007/05/darkest_chocola.html based on a Martha Stewart recipe. It's very fussy and quite a lot of work, but oh, it is impressive! Because the sugar in the French buttercream is melted into the egg yolks, it makes a smoother cream than American buttercream (which is made with confectioner's sugar). It will keep, tightly covered, in the refrigerator overnight, or up to 1 month in the freezer. Before using, let stand at room temperature until spreadable, then beat with electric mixer on high until smooth and fluffy again.