Our "floating island" is a shimmering meringue that's baked in a charlotte mold and surrounded by creamy pistachio custard. A tangle of crisp, pale-gold spun sugar is the finishing touch.
Preheat oven to 350 degrees with rack in lower third. Butter a 4 1/4-cup charlotte mold; set aside. Put egg whites into the bowl of an electric mixer fitted with the whisk attachment and beat on medium speed until frothy. Beat in cream of tartar. Gradually add sugar and beat until stiff and shiny, 8 to 10 minutes. Transfer meringue to prepared mold. Tap it on counter to remove air pockets. Butter a round of parchment; place, buttered side down, over meringue.
Put mold in a medium saucepan; transfer to oven. Carefully pour hot water into the pan to reach halfway up the side of the mold. Bake until meringue is puffed slightly over top of mold, 10 to 12 minutes. Transfer mold to a wire rack; remove parchment. Let cool 30 minutes. Meringue can be refrigerated in the mold up to 1 day.
To serve, unmold onto a platter; drain any liquid. Spoon creme around meringue, and top with sugar crown.