Preheat the oven to 350 degrees F and grease a 10-inch tart pan (with removable bottom) or 5-6 small tart pans with a non-stick spray. Set aside.
In a medium bowl, combine the graham cracker crumbs, granulated sugar, salt and butter. Mix thoroughly and then press it into the tart pan (or tart pans), making sure to cover the entire bottom as well as the sides. Press firmly so that the crumbs adhere. Refrigerate the shell for 30 minutes. Once chilled, bake the shell for 8-10 minutes or until the crust is brown and toasted. Remove from over and set aside to cool.
Reduce the oven temperature to 325 degrees F. To a medium saucepan, add the lime juice, granulated sugar, eggs and heavy cream. Whisk to combine. Cook over low heat, stirring constantly over medium-low heat, until the liquid is thick enough to coat the back of a spoon, about 9-10 minutes. Pour the mixture into the cooled crust and baked for 12-15 minutes, or until the filling just set. You can tell when the filling has set when you touch the center. If it doesn't stick to your finger, then the filling is done. Let the tart cool to room temperature and then transfer to the freezer for 30 minutes.
While your tart is chilling, prepare the meringue. Combine the egg whites, sugar and salt in a heatproof bowl. You will create a double boiler by placing the bowl over a barely simmering pot of water (make sure the water isn't touching the bottom of the bowl). Whisk the mixture constantly until the sugar has dissolved. Remove from heat and transfer to the bowl of your stand mixer, fitted with whisk attachment (or you can use a handheld electric mixer. Whisk the whites on high speed until medium peaks form. Stir in the vanilla extract.
Spread the meringue onto the tart (or tarts), leaving the edges uncovered. At this point you can brown the top with a kitchen torch or place in a 400 degree F oven for 2-3 minutes or until brown on top.
Serve immediately or chill until you are ready to serve.