The secret of how not to cry over buttercream.

For last weekend, when my dad was celebrating his birthday, I made him a Donauwelle. Which, in case you are not familiar with that very German cake, is a cake baked on a sheet, with dough a bit like marble cake (light part at the bottom, dark part on top of it) with added cherries. That sheet cake, after baking, is topped off with a layer of vanilla buttercream and covered with a chocolate glaze. Because the cherries sink into the dough when baking and take the dark dough with them, it looks kind of wavy from the side - hence the name.

So. I did mention the buttercream layer, right?

Buttercream. I love buttercream. That stuff and I, we also have sort of a history... and it took me a lot of time to actually understand it. You see, there are plenty of really old buttercream recipes in my old-fashioned books, and they are basically butter, whipped together with powdered sugar, and some liqueur added.

Then, at some point, people sort of figured out that you can make it a little lighter and less calorie-dense if you add custard. So then you get recipes for buttercream with custard... which, once upon a time, was thickened with help of egg yolks.

Today, usually, most recipes call for the more modern and more convenient type of custard - the kind you make by mixing a packet of starch-based powder with added cocoa or aroma into hot milk and getting it to the boil. That gives you a fair custard, but it can also lead to a lot of trouble when you start mixing it into butter for buttercream.

Buttercream is, basically, a water-in-fat emulsion. Like all emulsions, it depends on emulgators for stability - and butter, on its own, only has a very limited amount of these. There will be a moment when you add a spoonful too much of the custard, which counts as water for these purposes - and your buttercream will break and separate into a grainy mass. Uh oh.

Now... egg yolk is an emulsifyer - and a really, really powerful one if it's raw. One egg yolk, according to Harold McGee's "On Food and Cooking - The Science and Lore of the Kitchen" is enough to make several litres of mayonnaise (which is an oil-in-water emulsion). Egg yolk loses a lot of its potency when you heat it up, but it will still pull some weight.

So making an older-fashioned buttercream with egg as a thickener for the custardy part will more probably result in non-grainy buttercream even if you add quite a bit of stuff to the butter. The starch-based custard, though... it brings nothing to the marriage. Well, nothing but trouble.

It took me one long afternoon of beating buttercream to grainy horribleness, starting over with fresh butter and repeating the process of breaking it and then cursing, until I finally started to think. And at long last, I realised I was breaking down the emulsion because I added too much liquid, and that I obviously need more emulsifyer if I want to add more than just a very little bit of custard (the Donauwelle recipe I have tells you to beat almost a full litre of milk's worth of custard into 300 g of butter).

So these days, I have a secret weapon. These days, when I bake the cake to go with the buttercream, I take a little bit of one egg yolk and put it to the side. About half a teaspoon is plenty - that much, or even a full teaspoon, will not be missed in the cake and it will not be noticeable in the buttercream, except by the good behaviour of the latter. Beat up the soft butter (room temperature or a bit warmer), beat in the bit of egg yolk, and then add the custard one spoonful at a time, beating it into the butter. I usually leave a little bit of custard in the pot when I get the feeling that it's enough now, but since I add the bit of yolk, I've never had a buttercream go grainy on me again, and I have had no trouble at all with adding warm custard to cooler butter either.

So there you go. May all your buttercreams be delicious and firm and buttery, and never go grainy on you.