Those of you who have a hang-up about dietary fat, plug your ears. Classic crème mousseline is a 2-1 (by volume) mixture of pastry cream and cool butter, whipped together into a rich, silky foam. It could only have been invented by the French, God love those people. If you want you can substitute stabilized whipped cream in your fraisier, but really…do you want to miss this? No, I thought not.