Magic Key Lime Pie Photo by Goran Kosanovic for The Washington Post
— 8 to 12 servings

Adapted from "Bravetart: Iconic American Desserts" by Stella Parks

Ingredients 
For the sweetened condensed milk
4 cups milk (any fat percentage will do)
3/4 cup heavy cream
1 cup sugar
1/8 teaspoon kosher salt

For the pie
1/2 cup egg yolks (from about 8 large eggs)
2 tablespoons finely grated lime zest (no pith), plus 3/4 cup juice (from 6 to 9 limes)
1 homemade or store-bought cracker crust 

For the topping
1/4 cup sugar
Pinch kosher salt
3/4 cup  heavy cream

Method

For the sweetened condensed milk
Combine the milk, heavy cream, sugar and salt in a 5l pot over medium heat; cook, stirring occasionally with a heat-resistant spatula to help the sugar dissolve, about 12 minutes.
Continue cooking for an additional 30 minutes, scraping continuously to prevent a milky buildup from forming around the sides. 
Once the thickened milk-syrup suddenly begins to foam, it's almost done. 
Keep cooking until the foam subsides and the dairy has condensed to exactly 2 cups.
 Pour into an airtight container, seal to prevent evaporation and refrigerate up to 1 month. 
To mimic the consistency of canned milk, bring to room temperature before using. 
You'll need all of it for this recipe.

For the pie
Whisk together the egg yolks, lime zest, juice and the 2 cups of sweetened condensed milk in a 3l pot. 
Cook over medium-low heat, whisking gently to form a custard that is steaming hot, about 5 minutes. Increase the heat to medium; cook until the custard begins to bubble, about 5 minutes more. (It won't seem terribly thick.)

Pour into the prepared crust and refrigerate uncovered until cold and firm, at least 4 hours. (Covered loosely in plastic at this point, the pie can be refrigerated for up to 24 hours.)

For the topping
Combine the sugar, salt and cream in the bowl of a stand mixer fitted with the balloon-whisk attachment. 
Beat on medium-low speed for a minute to dissolve the sugar, then increase the speed to medium-high; beat until the cream is thick and forms stiff peaks, about 5 minutes. 
Transfer to a pastry bag fitted with a large tip; pipe decoratively on the pie.

Wipe the serving knife clean between cuts.

The Washington Post