Crème mousseline — also known as German buttercream — is a silky and decadent combination of pastry cream and butter. It’s often used as a filling, though it works just as well as a frosting, as the “buttercream” moniker implies. The proportions for crème mousseline are 2 cups pastry cream to one cup very soft butter. Yeah, I know. Wow.
Here I am adding one cup of unsalted, cultured butter to 4 cups of pastry cream that I made with the full amount of sugar and twice the vanilla. I’m getting light-headed already.
I beat that in for about three minutes and it’s looking a little curdled, which is OK. So I’m pressing on with the second cup. Steady me.
After another three minutes or so of beating, this is the result:
A perfectly smooth and glossy mousseline/buttercream. Should yours still be lumpy, it could be because the butter was too cool when you started. That’s not a problem. Just let the mixture sit and warm until the butter softens completely, then beat — or whip — the mixture some more. In extreme cases where you have lots and lots of cool butter pieces, it’s OK to heat the mixture a little. Put it in a microwave-safe container and zap it for five seconds, stir, zap another five seconds until the mousseline is slightly warmed and the butter is utterly softened. Some butter may separate out when you do that, which is completely fine. Just dump the whole mess back into your stand mixer and apply the whip until it fluffs up as seen above.
For those who are curious about buttercreams but worry about syrup making, crème mousseline makes an excellent intro to the genre. You just combine everything and beat it. It’s also a nice way to recycle leftover pastry cream because the addition of lots of butter makes crème mousseline freeze-able. You’ll need to re-whip it once it thaws, but that’s no big deal. Overall it’s an eggier taste than a standard buttercream, but since when was that ever a problem?
I should add that some German buttercream recipes call for double more butter, up to double. That’s too much for me. You’d need to revive me with smelling salts.