Bring a medium pot filled half way with water to a simmer. In a medium stainless-steel bowl that fits over the pot without touching the water, whisk the eggs, yolks, zest, sugar, and lemon juice. Put the bowl over (not touching) the simmering water and whisk until the mixture thickens and becomes smooth and custard-like, about 10 minutes; remove from the heat. Whisk in the butter a piece at a time. Strain the curd through a fine mesh into a bowl. Put plastic wrap directly on its surface and refrigerate.

In a medium, heavy-based saucepan, combine the sugar, corn syrup, and water; stir until the sugar dissolves. Cook over high heat until the mixture turns dark amber. Don't stir the caramel while it cooks; instead, swirl the pan gently to get an even color. Remove from the heat and whisk in the cream (be careful: it will splatter). Return the mixture to the heat, whisk until smooth, and then whisk in the vanilla. Let cool to room temperature.

Spray a 9-inch springform pan with nonstick cooking spray or grease it lightly. Dust the pan with sugar and knock out any excess. Combine the toasted graham cracker crumbs and butter, rubbing them together with your fingertips to combine thoroughly. Sprinkle half of the crumbs over the bottom of the pan and pat down; reserve the rest.

In a large bowl, whisk the cream to firm peaks. Fold in the cooled lemon curd. Spoon half of the lemon cream over the cracker crust and spread it evenly to the edges of the pan. Sprinkle the remaining crumbs over the lemon cream. Spread the remaining lemon cream over the crumbs. Pour a little more than half of the caramel over the lemon cream, reserving the rest in the refrigerator. Put the cake in the freezer while you make the meringue topping.

Whisk the egg whites and sugar in a double boiler over medium-high heat (as you did with the lemon curd) and cook until the mixture is warm and the sugar is dissolved, about 2 minutes. With an electric mixer, whisk the whites to stiff peaks. Spread the meringue on the top of the cake. Freeze the cake, unwrapped, overnight. (For longer storage, wrap it in plastic once the meringue has firmed up; unwrap before defrosting.)

About an hour before serving, transfer the cake to the refrigerator. Just before serving, reheat the remaining caramel sauce if you want to drizzle some on the plate. Brown the meringue by running it under a hot broiler, rotating the cake if necessary, until evenly browned (or brown it with a propane torch). Run a thin knife around the sides of the cake and remove the springform. Cut the cake into slices with a warm knife. If the cake seems very frozen, let the slices soften somewhat before serving. Serve with a drizzle of warm caramel sauce.