It is believed that French buttercream was first invented in 1865 by a French baker called Quillet and his first version was made with egg yolk. There have been so many variants since then, but we are not about to waste anything here so we will use the whole egg – I’m sure you have enough whites in your freezer by now! The egg is cooked by the hot sugar syrup so don’t be alarmed at the prospect of eating raw egg. You could bake this genoise in a baking tray and turn it into a yule log at Christmas time; if you like, you can add a little brandy to the coffee syrup for a bit of festive oomph.