The foundation of this recipe is grapefruit curd, a classic cooked sauce that can be used as a spread on toast as well as a dessert component. The curd can be made in advance and kept for up to a week in the refrigerator. When you want to make the grapefruit mousse, simply whip the cream and fold it into the curd, then garnish with our pretty Berry Sauce and mint sprigs for a light, quick treat.
Place the grapefruit juice in a small saucepan over medium heat and reduce to 3/4 cup. Transfer the juice to the top of a double boiler and add the sugar and egg yolks, whisking to combine. Set the double boiler over simmering water (low to medium heat), and cook the mixture until it thickens, stirring constantly, about 10 to 15 minutes. The curd should thicken enough to coat the back of a spoon. Don't let the mixture boil.
Strain the mixture through a sieve into a clean bowl. Whisk in the butter and the grapefruit zest. Place a round of waxed paper or plastic wrap directly onto the surface of the curd and refrigerate until completely cold.
Place the heavy cream and vanilla in a clean, chilled bowl. Whip the cream until it holds stiff peaks. Add about a third of the whipped cream to the grapefruit curd, whisking to combine the mixture. Then, carefully fold in the remaining cream.
Spoon or pipe the grapefruit mousse into glass bowls or goblets. Refrigerate until serving time. The mousse will hold overnight, although it's best served within 8 hours. Just before serving, drizzle with berry sauce and garnish with mint sprigs.