Crush the crackers in a plastic bag using a rolling pin or a heavy pot. Place into a large mixing bowl and add the sugar, then knead in the butter until the crumbs hold together like dough. Press into an 9-inch pie pan. Chill for 15 minutes.
While the crust is chilling, preheat the oven to 350 degrees F.
Bake the crust for 18 minutes and then set on a rack while you make the filling. Keep the oven on. Do not make the filling until the crust is done. It will begin to set while it sits.
To make the filling, combine egg yolks, sweetened condensed milk and lemon juice. Mix until smooth with electric mixer on medium speed or a large whisk.
Spread filling into crust; the crust does not have to be cooled.
Now we make the meringue. Beat egg whites with cream of tartar and salt on medium-high speed with an electric mixer for 1-2 minutes, or until soft peaks form when beaters are raised.
Gradually beat in sugar, 1/2 tablespoon at a time, until stiff glossy peaks form, and sugar is dissolved.
Spoon meringue onto filling and spread to edge of crust to seal well and prevent meringue from shrinking.
Bake pie for 15-17 minutes or until meringue has set.
Cool for 30 minutes at room temperature, then refrigerate for 4-6 hours or until pie is completely cold.