Heat the oven to 350°F.
In a medium bowl, combine the graham cracker crumbs, sugar and salt and mix until combined. Add the butter and mix until the crumbs are well coated (I find that it is best to use your hands to mix everything here; just be sure you allow the butter to slightly cool). Press the crumbs into the bottom and up the sides of a 9-inch pie dish. Bake the crust for 10 minutes, or until it is lightly browned. Set on a cooling rack.
While the crust cools, beat the lime zest and the egg yolks with an electric mixer (I use a hand mixer for this recipe, which works just fine) until the mixture becomes pale and thickens up, which should take a solid 5 minutes. Add the sweetened condensed milk and beat until the mixture becomes thick again, about 3 minutes. Whisk the lime juice into the yolk mixture until combined. Pour the filling into the crust and bake for 15 minutes, or until the filling is set and a knife inserted into the center comes out nearly clean (though not entirely clean). Remove the pie from the oven and set on the counter to cool.
While the pie cools to room temperature, beat the cream and sugar in a medium bowl until soft peaks form. Spread the whipped cream on top of the cooled pie and then chill the pie in the fridge for 2 hours before serving.