Preheat oven to 350 degrees F.
Combine crushed crackers and sugar and stir until well mixed, then add the butter and stir until the crumbs hold together. Press into an 8 or 9 inch pie pan. Chill for 15 minutes, then bake at 350 for 12-15 minutes or until nicely browned. I used a 9 inch pie pan and my crust was done in about 12 minutes.
Whisk together condensed milk, and egg yolks, then whisk in the juices and zest. Pour into the baked shell and bake for another 16 minutes or until filling appears set (with the 9 inch pan it still took 16 minutes).
Let the pie cool, then chill thoroughly for a few hours.
Shortly before serving, make the sweetened whipped cream, put the cream in a mixing bowl and beat until it starts to thicken. Add the sugar and continue beating until thick (do not overbeat). Stir in about ½ teaspoon of vanilla if desired.
Pipe the cream around the edges or spread over the top. Garnish with sea salt if desired.