1 cup + 2 tablespoons finely ground graham cracker crumbs
(I used low-fat ones. Probably a pointless idea with the other decadent ingredients.)
1/3 cup sugar
5 tablespoons butter, melted
3 large egg yolks
1 1/2 teaspoons finely grated lime zest
2/3 cup fresh Key lime juice , about 23 Key limes total
(25 key limes gave me just over 1/3 cup, but it worked fine.)
1 14-oz. can sweetened condensed milk
Stir together graham cracker crumbs, sugar, and butter in a small bowl. Press evenly onto bottom of an 8-inch square glass baking dish. Bake at 350 degrees until dry and golden brown, about 10 minutes. Let cool completely.
(Warning: there's lots of mixing required.) To make filling, put egg yolks and lime zest in the bowl of an electric mixer fitted with the whisk attachment. Mix on high speed until very thick, about 5 minutes. Reduce speed to medium. Add condensed milk in a slow, steady stream, mixing constantly. Raise speed to high; mix until thick, about 3 minutes. Reduce speed to low. Add lime juice; mix until just combined.
Spread filling evenly over crust using a spatula. Bake, rotating dish halfway through, until filling is just set, about 10 minutes. Let cool completely on a wire rack. Refrigerate at least 4 hours (or overnight).