Set out a 9-inch pie pan and a food processor. Place the graham crackers in the food processor. Lock the lid into place and pulse into a fine crumb. Then add in the melted butter, sugar, and salt. Pulse to combine.
Dump the graham cracker crumble into the pie pan. Use your hands to press it into an even layer over the bottom of the pan and up the sides. Refrigerate until ready to fill.
Wipe out the food processor bowl. Then add in the cream cheese, sweetened condensed milk, lemon juice and lemon zest. Puree until smooth. Pour/scoop the filling into the pie crust and continue to refrigerate.
In a medium bowl, mix the sour cream and powdered sugar. Stir until smooth. Then fold in the Cool Whip. Spread the topping over the lemon pie filling and place in the freezer for at least 4 hours, undisturbed.
When ready to serve, cut into pieces while frozen. Let each piece sit out at room temperature for 10 minutes, then serve slightly softened. Garnish with lemon sliced if desired.
Make the Pie as a Single Layer: If you want one thicker layer of lemon filling, with no topping, omit the powder sugar and sour cream. Then fold the cool whip into the lemon pie filling before place in the crust. Prepare as a Key Lime or Lime Icebox Pie: Simply swap the lemon juice for fresh lime juice.The lemon ice box pie will stay fresh in the refrigerator for 4-5 days. Cover the pie with plastic wrap before putting in the fridge, and cut the pie while still cold to serve.To freeze, cover the pie well with plastic cling wrap and then a layer of aluminum foil. Keep in the freezer for up to 3 months.