A classic cool and tart southern desert that hits the spot all summer long. Since Key limes aren't widely available, we use regular supermarket (Persian) limes, and you could also use bottled key lime juice. In theory, you could use another citrus fruit entirely (pamplemousse tart, anyone?), though it's interesting how the sweetened, condensed milk contains just enough sugar to balance lime juice. You could simplify the procedure considerably by purchasing a pre-make graham cracker shell.
1-1/4 cups all-purpose flour
1 stick unsalted butter, chilled and cut into small pieces
2 teaspoons sugar
1/4 teaspoon salt
About 3 or more tablespoons of ice water
4 teaspoons grated lime zest
4 eggs, separated into yolks and whites
1 can (14 ounces) sweetened condensed milk
1/2 cup fresh key lime juice (4+ limes)
Whipped cream topping:
1-1/2 cups chilled heavy cream
1/2 cup powdered sugar
Make the crust dough. Add the flour, chilled butter, salt, and sugar to the bowl of a food processor. Pulse repeatedly until the mixture has the texture of coarse crumbs. Add the ice water and pulse until the dough just starts to stick together. Turn the dough onto a floured surface. Form the dough into a flat disc, wrap in parchment or wax paper, and refrigerate for an hour.
Make and bake the pie shell. Roll out the disc of dough to a 12-inch circle. Transfer to a ~9-inch pie pan. Trim and crimp the edge, and prick the bottom of the crust bottom a dozen times. Chill for 20-30 minutes, or up to 24 hours. When you're ready to bake, preheat the oven to 350F. Line the crust with parchment paper, then fill with pie weights (or beans or rice). Bake until slightly underdone (we'll be baking with filling later), about 30 minutes. Remove weights and parchment and cool.
Make the filling. Whisk the lime zest and egg yolks together in a medium bowl for 2 minutes. Whisk in the sweetened condensed milk and the lime juice. Set aside at room temperature to thicken a bit. Use an electric mixer and beat the egg whites to soft peaks, then fold into the lime juice/condensed milk mixture. Pour the filling into the cooled crust.
Bake until the filling is mostly set, about 20 minutes. Remove pie from oven and cool thoroughly.
Make whipped cream. Using an electric mixer, whip the cream to soft peaks. Add the powdered sugar in several small batches while continuing to whip the cream, until the stiff peaks stage is reached. Spread the whipped cream evenly over the cooled pie. Refrigerate until ready to serve.