In case you missed it, yesterday was Pi Day in honor of that famous irrational number 3.14152965… The staff of Magrath Library celebrated with a pi(e) potluck (highly recommended, even for the non-mathematically-inclined). I took this opportunity to try out a recipe from the Kirschner Collection that is unlike any pie I’ve ever made before. Mrs. Foster’s Lime Pie from Maida Heatter’s New Book of Great Desserts is a light, airy, frozen pie that would just be perfect on a hot summer day (which incidentally, is exactly what I’m dreaming of right now). Topped with some whipped cream, it was a big hit at the potluck. 

Mrs. Foster’s Lime Pie

Crumb Crust

1 1/4 cups graham-cracker crumbs
1/4 cup granulated sugar
2 ounces (1/2 stick) unsalted butter, melted

Filling

5 eggs (graded large), separated
3/4 cup granulated sugar
2/3 cup fresh lime juice (grate the rind of 2 limes before squeezing, to use below)
Finely grated rind of 2 limes
1/8 teaspoon salt

Crumb Crust

Adjust a rack one-third up from the bottom of the oven and preheat oven to 350 degrees.
In a bowl, mix the crumbs with the sugar. Then add the butter and stir with a rubber spatula until the butter is evenly distributed; the mixture will not hold together.
Turn the mixture into a 9-inch ovenproof glass pie plate. With your fingertips loosely distribute the crumbs evenly over the bottom and sides of the plate. Then press firmly, first on the sides and then on the bottom, to form a compact crust.
Bake for 10 minutes and then cool to room temperature. While the crust cools, prepare the filling.

Filling

Adjust rack one-third up and preheat oven to 350 degrees.
In the small bowl of an electric mixer, be the yolks with 1/2 cup of the sugar, reserving remaining 1/4 cup of sugar, at high speed for 5 minutes until the mixture is very pale and thick.
On low speed gradually add the lime juice, scraping the bowl with a rubber spatula and beating only until mixed.
Remove from the mixer and stir in the grated rind.
Turn the mixture into the top of a large double boiler over shallow hot water on moderate heat (the water should simmer gently). Cook, scraping the sides and bottom constantly for 6 or 7 minutes or until the mixture thickens enough to coat a wooden spoon. When it is just done, a candy thermometer will register 175 to 180 degrees.
Remove the top of the double boiler immediately and pour the mixture into a large mixing bowl. Stir occasionally until it cools.
In the large bowl of an electric mixer, add the salt to the egg whites and beat until they hold a soft shape. Reduce the speed to moderate and gradually add the reserved 1/4 cup of sugar. Increase the speed again and continue to beat until the mixture holds a definite point, but not until it is actually stiff or dry.
Gradually, in three additions, fold the yolk mixture into the whites, handling very little and very carefully. It is not necessary to fold the first and second additions thoroughly.
Turn the mixture into the crust. It will look like there is too much filling, but it is all right, just mound it high. (It will be about 4 inches high in the middle.)
Bake for 15 minutes until the top is lightly browned.
Cool on a rack to room temperature. (The pie will shrink slightly as it cools — O.K.) Then freeze the pie. When it is frozen, cover it with plastic wrap. The pie may be kept frozen for up to 3 weeks. Serve it frozen with whipped cream.

By Maida Heatter’s New Book of Great Desserts (1982)