Well, both FBC and GBC are egg-yolk-based buttercreams or custard-based buttercream. The difference between the two is in the method we use. - In French buttercream, we use the method of boiling sugar syrup to 240 F then pour it over whipping eggs. -While in a German buttercream, we first make the vanilla pastry cream using egg yolks and then combine it with butter whipping it until we have a luscious buttercream. - The French method requires a thermometer and this German method does not.