Line a pie plate with the crust, crimp the edges, dock it with a fork, and refrigerate the crust for at least 30 minutes. Line the crust with foil and pie weights (or dry beans) and parbake it for 15-20 minutes at 350°F. Remove the pie weights and foil and continue baking until golden brown, another 5-10 minutes. Set aside while you prepare the filling.
In a large saucepan, combine the milk and cornstarch and whisk until the cornstarch is dissolved. Add the condensed milk, eggs, and lime juice, and whisk to combine. Bring to a boil over medium heat, whisking constantly. It will thicken considerably when it boils. Boil for 1 minute, whisking constantly. Remove from the heat, then whisk in the butter and lime zest.
Place the saucepan into a larger bowl filled with ice, and stir the filling occasionally until it is cool, about 10 minutes. Pour into the prepared crust, level it off, and refrigerate for an hour or so to set.
Make the “Jello shot” layer by combining the lime juice, water, and sugar in a saucepan, and sprinkle the gelatin over the surface to bloom. When the gelatin has bloomed, bring to a simmer to fully dissolve the gelatin. Remove from the heat and whisk in the zest, tequila, and food coloring if you’re using it. Once it’s cooled and beginning to thicken, pour over the filling already in the pie. Return the pie to the fridge for this layer to set completely, which will take an hour or so.
Finally, make the meringue for the topping. Put the egg whites and cream of tartar in the work bowl of a stand mixer fitted with the whisk attachment (make sure the bowl and whisk are impeccably clean; and fat or grease will spoil the meringue.) Combine the water and sugar in a medium saucepan with a candy thermometer and put over medium-high heat. Bring to 240°F. A bit before (20° or so early) the syrup reaches the target temperature, start whisking the egg whites. When they reach soft peaks, turn the mixer off.
When the syrup reaches the target temperature, immediately remove from the heat, turn the mixer back on at medium speed, and pour the syrup carefully in a very thin stream into the mixer bowl. Aim for the spot just outside where the whisk hits the bowl; you don’t want the whisk to fling hot syrup all over the place, just incorporate it slowly. When the syrup is completely added, add the vanilla and continue to whisk at medium-high speed until the exterior of the work bowl no longer feels warm to the touch, 8-10 minutes.
Using a rubber spatula, cover the entire pie with a dome of meringue, making sure to contact the crimped edge of the crust all the way around. Lightly toast the outside of the meringue with a pastry torch or other blowtorch.
Because this is an Italian meringue (cooked meringue) this will be pretty stable, so it will hold for a bit if you’re not serving right away.