Set an oven rack on the center shelf, then preheat the oven to 325 degrees F.
If using graham cracker sheets instead of crumbs, process them into fine crumbs using food processor (You can also place them in a bag and smack them using something like a rolling pin or the bottom of a saucepan, until you have fine crumbs).
Combine graham cracker crumbs, sugar, and salt in a small bowl. Add the butter and stir with a fork until evenly combined. Press the mixture firmly into a shallow 9-inch pie pan, pressing the crumbs up the sides of the pan but avoiding the lip. You can use the bottom of a glass to help press them down firmly, or use your hands (I use kitchen-safe gloves to keep the mixture from sticking to my hands).
Bake until the crust is fragrant and slightly darker in color, about 15 minutes. Transfer the pan to a cooling rack and cool to room temperature, about 20 minutes.
While the crust is cooking, prepare the filling. Whisk the yolks and zest in medium bowl until the yolks lighten up slightly in color, possibly with a slight green tint, about 1 minutes. Whisk in the condensed milk followed by the juice; let sit at room temperature until crust is cool (this will help the flavors permeate and the mixture will thicken very slightly).
Pour the filling into the crust, smooth out the top, and bake until the center has just set but is still slightly wobbly, 14 to 17 minutes. Transfer the pie to a cooling rack and cool to room temperature. Refrigerate until chilled, at least 3 hours or overnight.
Prior to serving (up to 2 hours in advance), whip the cream in a medium bowl until you it’s at a very soft peak. Whisk in the confectioners’ sugar, 1 tablespoon at a time, then continue whipping to just-stiff peaks. Spread whipped cream evenly over the pie with a rubber spatula (or you can spoon onto individual slices). Garnish with grated lime zest, and enjoy within 2-3 days.
Using real key limes is always the best choice, but they’re not always going to be available. Regular limes work fine, as well as bottled key lime juice (I used Nellie and Joe’s brand). Don't use bottled regular lime juice. That stuff always leaves a bad aftertaste.I recommend using a microplane zester to grate lime zest.If whipped cream is over-whipped, it will look grainy (even though it tastes fine). Read How To Make Whipped Cream for tips on how to get perfect results every time!Adapted from Cook’s Illustrated Magazine (March 1997 Issue)