How to Make It
Bring a medium saucepan filled with 2 inches of water to a simmer over moderate heat. In a medium heatproof bowl, whisk the grapefruit juice and zest with the 6 whole eggs, 6 egg yolks and 3/4 cup of the sugar. Set the bowl over the saucepan and cook, stirring frequently, until a thick curd forms, about 15 minutes; don't worry if the curd looks slightly curdled. Strain the curd through a fine sieve set over a bowl, pressing on the solids; discard the solids. Whisk the softened butter into the curd until blended. Place a sheet of plastic directly on the curd and refrigerate until set, about 3 hours.
Meanwhile, in a food processor, crush the graham crackers; pour in the melted butter and pulse just to combine. Pat the crumbs over the bottom of a 9-inch springform pan. Refrigerate for 30 minutes.
In a medium bowl, using an electric mixer, beat the cream cheese until fluffy. Beat in 3/4 cup of the Caramel Sauce. In another medium bowl, beat the heavy cream until soft peaks form. Fold the heavy cream into the caramel cream cheese.
Spoon the grapefruit curd into the springform pan and tap gently to form an even layer. Spoon the caramel cream on top in a smooth, even layer. Drizzle the remaining 3/4 cup of Caramel Sauce all over the caramel cream and freeze until slightly set, about 1 hour.
Meanwhile, bring a medium saucepan filled with 2 inches of water to a simmer. In the bowl of a standing electric mixer, combine the 6 egg whites with the remaining 1 1/3 cups sugar, set the bowl over the simmering water and whisk over low heat until the sugar is dissolved and the egg whites are hot to the touch. Transfer the bowl to the mixer fitted with a whisk and beat at medium-high speed until the whites are stiff and glossy, about 8 minutes. Spread the meringue over the caramel. Swirl decorative peaks in the meringue. Freeze the pie until firm, at least 4 hours.
Before serving, preheat the broiler and position a rack 8 inches from the heat. Broil the meringue just until it begins to brown, about 2 minutes, shifting the pan for even browning. Alternatively, brown the meringue with a propane torch. Carefully remove the ring and transfer the pie to a platter. Let stand in the refrigerator for 30 minutes before serving.