Step 1Heat the oven to 325 degrees. Lightly coat a 9-inch pie plate (not deep-dish) with nonstick cooking spray.
Step 2Make the crust: Combine the graham cracker crumbs, sugar, salt and melted butter in a large bowl and stir until moist crumbs form. Transfer to the prepared pie plate and press evenly into the bottom and up the sides of the plate (but not over the rim).
Step 3Bake the crust until lightly browned, about 10 minutes. Transfer to a wire rack to cool completely. Leave the oven on.
Step 4Make the filling: Finely grate 1 tablespoon zest from the limes into a large bowl, then halve the limes and squeeze ½ cup juice. (You may not need all 6 limes.) Add the condensed milk and egg yolks and whisk until all the egg yolks have been incorporated and the zest is speckled throughout the mixture. Pour into the cooled crust.
Step 5Bake until the filling is set around the edges and the center wobbles slightly when touched, about 15 minutes. Transfer to a wire rack and let cool completely, at least 2 hours.
Step 6In a large bowl, whisk the heavy cream and sour cream with an electric mixer fitted with the whisk attachment, or by hand, until soft peaks form; do not overbeat. Pile the cream on top of the pie and swirl and swoosh with a spoon or rubber spatula. Refrigerate the pie for at least 3 hours before serving.
Key lime slab pie
Double all of the quantities above. Use a 9-by-13-inch cake pan or quarter-sheet pan instead of the pie plate and prepare and bake the crust and filling as above. To serve, cut into squares and top each with a large dollop of the cream mixture.