Four years ago,
when I first posted about this cake, I had a tempestuous relationship with the icing.
Behind the scenes, the icing in that post caused a fit of frustrated weeping (which was very embarrassing, as my then-brand-new-now-ex boyfriend was staying with me at the time - and let's face it, I was probably trying to impress him). Luckily I left it mixing and a few minutes later, it had whipped itself back.
Mum and I had been making the cake for years - it was my granny's favourite - but because we didn't understand the reasons and techniques behind the recipe, the icing was always nerve-wracking to make.
Delia calls the icing 'mousseline' (which appears to be a catch-all term for various things enriched with butter or whipped cream) but I think it's more accurate to call it French buttercream. One of our first exam dishes at Cordon Bleu involved French buttercream - it's a lot like Italian meringue buttercream except it's based on egg yolks instead of whites. I didn't realise the icings were the same until I made the cake for the first time in a few years for
mum's birthday in April. I also found that my old recipe was too brief and not very clear, so I thought this was a good opportunity to talk about French buttercream and rewrite it.
The first step is getting the syrup to the right temperature. I'd recommend testing with a thermometer because it's easier and the finger test takes time. Having said that, if you don't have a thermometer or want to try it out, it's quite fun. The test for thread stage is to spoon a bit of the syrup onto a plate, then wait for it to cool. Squidge some syrup between your thumb and first finger then pull them apart slowly until you have a gap of about a centimetre - a thread of syrup should stay linking the two.
One of the problems is that different recipes take the syrup to different temperatures. It doesn't help that the temperatures stated for the stages seem inconsistent too - for instance, we were taught that soft ball was 116-122C but McGee says it's 113-116C. Thread, similarly, is 105-110C or 102-113C. Though others go to soft ball, Delia recommends 103-5C for this - I've been taking it up to 110C and that has been working well, so I've settled for that.
The second step is to pour the syrup down the bowl into the whisking egg yolks. I found it quite hard to visualise at first and mum used to get into a tangle by pouring the syrup into the whisk and it spinning everywhere, so I decided to do a little video for you. Excuse the little slip of the pan near the end - I'd recommend using a smaller saucepan with a long handle as they're easier to control.
After that, you leave it to whisk until cool. It's really important that the mixture is room temperature before you start adding the butter or it'll melt and create a mess.
Once it has cooled, you add the butter bit by bit. To illustrate the speed to add it (and because I find watching the mixer going round mesmerising), here's a clip of the middle of the step. Also note that I often squish the cube I'm about to add to give it a last minute bit of warmth and pliability. The butter needs to be room temperature but not melting or greasy.
Once the butter is all in, give it few more moments to whip. Finally, you can add flavourings - in this case, coffee.
If it goes a bit curdled towards the end, this can usually be fixed by whipping it for longer. If not, try putting it in the fridge for 15 minutes then whipping again.
Though the icing is rich, I like it much more than normal buttercream. The bitterness of the coffee and touch of salt cuts through the limited sweetness. It's also very smooth and thick, which is particularly lovely against the crispy-edged cake and toasted walnuts.
When I was making the cake in the photos the other day, I realised I'd run out of unsalted butter when I needed to start warming it up for the icing. I had some salted in the fridge so I decided to try it. The icing finally tasted exactly like I remember it.
It reminded me that mum always used to use salted butter for her baking - partly because that was what was in the fridge - but also because it was apparently harder to buy unsalted when I was little. It made me wonder if something got lost in translation as unsalted became more popular, as many modern baking recipes use unsalted and don't add salt. Perhaps it's because people weren't used to having to add extra salt to sweet dishes as traditionally the butter was salted to preserve it. I guess it's also because of health concerns.
Generally the reason unsalted is preferred is so that you can control the amount of salt. I think a pinch of sea salt improves nearly all baking and desserts. It's useful to have the control as packets vary - my salted butter had 1.8% salt, which is therefore 2.7g in the 150g butter I used. It is quite a bit and while I personally think it tastes amazing in the icing, it would be too much in some other recipes.
Coffee and Walnut Cake
(adapted from Delia Smith's Book of Cakes)
For the cakes:
60g walnut halves + 12 for decoration
1 tbsp instant coffee* + 1 tbsp boiling water
110g unsalted butter, at room temperature
110g caster sugar
2 eggs
110g plain flour
1.5 tsp baking powder
pinch of fine sea salt
For the icing:
2 egg yolks
pinch of salt (or a little more, to taste, or use salted butter)
60g caster sugar
60g/ml water
150g unsalted butter, cubed, at room temperature
1 tbsp instant coffee* + 1 tbsp boiling water
Preheat the oven to 170C/340F. Toast all of the walnuts for 5 minutes then remove the 12 halves for decoration and chop the rest roughly. Grease then line the bottom of two 7" round tins. Combine the coffee and water in a bowl or mug. Cream the butter and sugar together for several minutes until very pale and fluffy. Meanwhile, sift the flour, baking powder and salt together. Beat the eggs lightly together in a jug or bowl then add small amounts to the mixer, beating well in between each addition. When all the egg is incorporated, add the flour and mix on low until combined. Add the chopped walnuts and coffee and fold through. Divide between the tins and smooth out - it will feel like there isn't much mixture but don't worry. Bake for about 25 minutes until a deeper, rich brown and a toothpick can be removed cleanly from the middle. Leave to cool for 5 minutes then turn out of the tins and leave to cool on a rack.
For the buttercream, put the egg yolks and salt into the bowl of a stand mixer with the whisk attached and whisk briefly until broken up and a bit frothy, then turn off. Place the sugar and water into a small saucepan and place over medium heat with a sugar thermometer attached. Once the sugar has dissolved, turn up to medium-high, occasionally giving it a gentle swirl, until it reaches 110C/230F. This takes me about 5-6 minutes (the last 10 degrees are often the slowest). The moment the temperature hits, turn the yolks up to medium speed then steadily pour the hot syrup down the side of the bowl, resting the lip of the pan on the top of the bowl (see video above).
Turn the mixer up and leave to whip until you can't feel any heat in the mixture - this takes about 7 minutes (check if it is cool by touching the outside of the bowl, then if that is room temperature, testing the mix itself with a finger). If it is cool, start adding the butter, which should be soft and pliable but not greasy. Add the butter cube by cube, letting each piece disappear before you add the next. At stages it might start looking weird or curdled but just keep whipping and slowly adding the butter - this all takes about 7 more minutes. When you've added all the butter and have a thick, glossy buttercream, whip in half the coffee, followed by the other half.
Place one of the cakes on a serving plate. Spread half of the buttercream over the cake then place the other cake on top. Spread the remaining buttercream over the top then decorate with the remaining walnut halves.
(Makes 12 small slices)
* This cake tastes weird to me with anything other than cheap instant coffee as that's what I grew up with. Feel free to try using very strong proper coffee, though I'm not quite sure how you'd get a thick concentrate like this.
