Yesterday I attended a half-day patisserie workshop based around choux pastry. In the course of it, we made some crème mousseline to fill our choux buns. This was a new one for me: it’s basically crème pâtissière with an unfeasibly large amount of butter beaten into it, resulting in a cream that is both airy and rich, and will not collapse under load. It’s apparently the basis for such treats as fraisiers and tropéziennes. Useful as a filling because its firmness means it won’t squelch out or drip when cut or bitten into. But it is very, very calorific, so special occasions only!
Best used on the day it’s made; it will go solid if refrigerated for more than a couple of hours because of all the butter. Apparently that can make it go grainy, but this can be fixed by putting it in the bowl of a stand mixer, starting the whisk at high speed, and then gently and briefly warming the outside of the bowl with a blowtorch (this is clearly a standard technique as our chef/instructor did this when the butter was too hard!). You could make this without a stand mixer (people obviously did in the past) but it’s a lot of work: much vigorous beating required.
The recipe below makes a massive amount, enough to fill at least 15 choux buns.
750 g milk
220 g sugar
100 g egg yolks (about 4-5)
90 g cornflour
1 tsp vanilla extract
100 g cold butter
150 g soft butter
Well before you start, get the butter that needs to be soft out of the fridge. It needs to be really soft.
First make the crème pâtissière. Put the milk in a pan and bring up to boiling point. While that’s happening, in a large heatproof bowl whisk together egg yolks and sugar until pale and creamy. Then whisk in the cornflour and finally the vanilla extract.
When the milk boils, pour it onto the egg yolk mixture, whisking to blend. Return the mixture to the pan and place over a moderate heat. Whisk constantly as it thickens, making sure it doesn’t catch on the bottom of the pan. Because of the cornflour, it’s fine — and even required — for it to boil. When it does, it will go lumpy. Don’t panic, just continue beating furiously over the heat until it is smooth and very thick.
Remove from the heat and immediately beat in 100 g of cold butter, cut into pieces. The mixture will become stiff and glossy. Line a large baking tin or tray with clingfilm and pour the cream onto half of it, spreading it out with a spatula. Then fold the clingfilm over to cover it and roll up to seal. Put in the freezer for 20 minutes or so to chill, then remove until needed. Note: you could do this in advance, for example the day before, leaving just the butter to be added on the day; in that case, put the cream in the fridge. Whichever way, don’t let it get too cold; if it’s really chilled, the butter might seize when you beat it in.
Put the soft butter in the bowl of a stand mixer and use the whisk attachment to beat it at high speed until it is really fluffy. This might take up to 5 minutes if the butter is not soft enough (it’s also when you might apply a blowtorch if you forgot to get the butter out of the fridge — it mustn’t melt or go liquid though).
Add the now fairly solid crème pâtissière to the butter. Then whisk furiously at high speed for 5-10 minutes, until the mixture is very airy and light. It’s now ready to use: pipe it into choux buns or use it to fill a fraiser or tropézienne.