Sky-high meringue is teamed with billows of freshly whipped cream and homemade rhubarb compote for this show-stopping dessert.
Preheat oven to 350 degrees. Lightly brush an 8-inch springform pan with coconut oil; line bottom with parchment. Beat egg whites on medium-high speed until soft peaks form. Slowly add 1 2/3 cups granulated sugar while beating continuously until stiff, glossy peaks form, about 7 minutes. Fold in cornstarch, vanilla, vinegar, and 1 pinch salt.
Spoon meringue into prepared pan, smooth top level with an offset spatula, and transfer to oven. Immediately reduce heat to 325 degrees; bake, without opening oven door, until cake has a pale honey color, 70 minutes. (If top appears to be getting too brown, lower heat slightly.) Turn off oven; let meringue stand inside, without opening oven door, at least 1 hour and up to overnight. (Marshmallow center will sink and leave high, crisp sides.)
Meanwhile, place rhubarb, remaining 2/3 cup granulated sugar, muscat, and remaining pinch of salt in a medium saucepan over medium heat. Simmer gently, stirring occasionally, until sugar is dissolved and rhubarb is tender and partially broken down, about 7 minutes. Transfer rhubarb to a bowl with a slotted spoon and continue to cook liquid until reduced by half. Pour liquid over rhubarb; let cool completely. Rhubarb can be made to this point and stored in an airtight container in refrigerator up to 1 week.
Just before serving, carefully remove cake from springform and place on a serving platter. Whisk together cream and confectioners' sugar until soft peaks form. Pile 2 1/2 cups whipped cream into center of meringue, then spoon 1 cup rhubarb mixture over. Serve, with remaining rhubarb mixture and whipped cream on the side.