Real Key Lime Pie Isn't Green. Nor is it baked.
I'm a total snob when it comes to this pie. It's got to be one of the easiest desserts ever, and whenever I go out and find some day-glo green whipped chiffon monstrosity masquerading as this pie, a little bit of me dies inside. There's just no excuse for ruining something so simple and so delicious. A good test for whether the pie is worthy of ordering at a restaurant is to ask how many ingredients are used in it. If the answer is more than four, then skip the pie - it's probably no good.
Key lime filling is really a very rich, uncooked pudding. Traditionally, this pie is done with a graham cracker crust but frankly, I think it's all wrong. The spices in the graham overwhelm and conflict with the fresh citrusy taste of the limes so here it's been replaced with a light shortbread crust that lets the tart, floral taste of the limes shine through.
Shortbread Crust (adaped from the Joy of Cooking, 1975)
1 C Butter
2 C Sifted All-Purpose Flour
1/2 C Sifted Powdered Sugar
1/4 tsp Salt
Fluted Tart Pan
Key Lime Filling
2 Cans Sweetened, Condensed Milk
6 Full Size Limes, or 8-12 key limes - enough to make a full Cup of lime juice.
Start with the crust. This is a VERY RICH PIE, so a thin little slice is about right - use a shallow tart pan instead of a full size pie tin for this one.
Mix the dry ingredients together in a large bowl.
Cream the butter (to soften it and make it easier to work with).
Add the creamed butter to the dry ingredients.
Depending on the temperature and humidity in your kitchen, the dough could look crumbly or come together in a smooth ball. Either is fine.
Press the dough into the tart pan, starting with the sides. Use your fingers to press it in, and keep it fairly thin - about 1/8 inch thick. Work it all the way around, then spread the rest of the dough to the same thickness on the bottom, using your fingers to press and seal it with the sides. Trim any excess from the top edge.
Shortbread Crust, full of little pricks and ready to get baked
Prick holes in the pie bottom to let the steam escape, and bake at 325 for 20-30 minutes, or until the crust is golden. Set aside to cool - you don't want cold pudding going into a hot crust or you'll get Key Lime Glop.
Once the crust is cooled, make the pudding. It only takes a few minutes, so don't start early. Tiny Key Limes are traditional for this pie, but they're a pain in the ass to work with and frankly, regular limes just taste better. I consider the "Key Lime" name to indicate where the pie hails from, not the ingredients.
The flavor is in the oils, and the oils are in the zest
Zest and juice the limes into a small ceramic, glass or plastic bowl. Making extra is a good idea, and don't skimp on the zest, it's the most important part.
Open the cans of milk and empty them into a large bowl.
Start by mixing half of the lime juice into the milk.
Taste it as you go and add more juice slowly.
As you stir, the lime juice and milk will react and thicken into a pudding before your very eyes! It's limey magic! As it thickens, add the rest of the lime juice and stir it in. If the pudding thickens and then starts to get runny, there's too much lime but you can add a little more canned milk to thicken it back up.
Pour the pudding into the crust, and voila! Done. It's really that easy.
Garnish with whipped cream if you like.
(As a side note, if you're having problems getting the pudding to set correctly, try adding 2 egg yolks to the mix and baking the pie at 350 for 15-20 minutes.)