Step 1Heat the oven to 350 degrees.
Step 2To make the crust by hand, place the crackers in a freezer bag and use a rolling pin to crush them into fine crumbs. In a medium bowl, combine the crumbs and sugar and toss with a fork to blend. Stir in the melted butter and toss to incorporate it. To make the crust using a food processor, process the crackers with the sugar until the crackers become fine crumbs, about 20 seconds. Add the melted butter and pulse just until incorporated, about 10 times.
Step 3Using your fingers or the back of a spoon, press the mixture into the bottom of the pie pan and partway up the sides. To keep the crumbs from sticking to your fingers, it helps to place a piece of plastic wrap over the crumbs and press them through the wrap. Then switch to a flat-bottomed, straight-sided measuring cup or glass to smooth the crumbs over the bottom and all the way up the sides. Be sure to press the bottom thoroughly so that the crumbs are evenly distributed. Place the crust in the oven and bake just to set the crust (the crust will color just slightly), about 6 minutes.
Step 1In a medium mixing bowl, lightly whisk the egg yolks with the sweetened condensed milk. Gradually beat in the lime juice. (It will cause the mixture to thicken.) Beat in the zest and set aside.
Step 2In the bowl of a stand mixer, or in a large mixing bowl with an electric mixer, beat the egg whites until foamy. Add the cream of tartar and beat until stiff peaks form when the beater is raised slowly. Set aside.
Step 3In a small, heavy saucepan, stir together the sugar and water until the sugar is completely moistened. Heat, stirring occasionally, until the sugar dissolves and the syrup begins to bubble. Stop stirring and reduce the heat to a gentle simmer and continue cooking until a thermometer inserted reads 236 degrees (soft ball stage). Immediately remove from the heat.
Step 4With the mixer running at low speed, gently pour the syrup in a slow, steady stream over the whites until the syrup is incorporated. Continue beating on high speed until the outside of the bowl is no longer hot to touch uncomfortably, about 2 minutes.
Step 5Remove a scant 2 cups of the meringue and cover the remainder with plastic wrap. Set it aside.
Step 6Gently fold the scant 2 cups of meringue into the lime mixture and pour it into the crust. Bake at 350 degrees for 15 minutes (to set the filling more firmly). Remove the pie from the oven and spread the remaining meringue on top of the filling, starting from the outside edge of the crust, covering the crust, and working toward the center of the pie. If desired, for extra crunch, dust the meringue with optional powdered sugar.
Step 7Return the pie to the oven for 5 additional minutes. Then turn the oven to broil and brown the meringue for 20 seconds to 1 minute, watching carefully to prevent burning, until the meringue is golden. Cool in a place away from drafts for at least 30 minutes, then refrigerate for at least 4 hours before serving. (The pie can be cut after 2 hours, but the slices hold their shape better after 4 hours.) Cut with a wet, thin-bladed knife. Store uncovered, refrigerated, up to 3 days.