1) Preheat the oven to 350°F. Lightly grease a 9" pie pan with non-stick baking spray.
2) Crush the pretzels until very fine; processing in a food processor for about 30 seconds works well.
3) Add the sugar and melted butter, mixing to combine; you can do this right in the food processor. Press the mixture into the bottom and up the sides of the prepared pan.
4) Bake for 8 to 10 minutes; the crust won?t change color much, but will firm up. Remove from the oven and make the filling.
5) Whisk together the egg yolks and condensed milk.
6) Add the lime juice (and zest, if you're using it). The mixture will thicken almost immediately.
7) Pour into the crust and smooth with a spatula.
8) Bake the pie for 28 to 32 minutes, until the filling is set, with a small spot in the middle still soft-looking. Cool for several hours (or overnight) before serving.
9) To serve, top with slightly sweetened whipped cream and a garnish of crumbled pretzels.
Yield: one 9" pie, 8 servings.