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.