Crème mousseline
Crème mousseline is pastry cream with butter beaten into it. The butter usually runs from a quarter to half the weight of the custard base, and that proportion decides how firm the finished cream is.
The critical condition is that both components be at the same temperature — cream and butter both at room temperature, around 20–22 °C. If the cream is colder than the butter the mixture curdles into grains; if it is warmer the butter melts and the cream slumps. A split mousseline can usually be rescued by warming the sides of the bowl slightly while continuing to whip.
The butter gives mousseline stability at room temperature and the ability to hold its shape, so it is used where the cream carries load: in a fraisier, in layers, in piping. It firms up markedly when chilled, so the finished dessert is left to come up in temperature before serving.