Any pie pastry will generally do. Roll the dough into a 13-inch circle and line a 9-½-inch deep-dish pie plate with it, forming the overhanging dough into an upstanding rim. Chill and partially prebake the shell. Then cool the shell.
Preheat the oven to 350°F. Combine the boiled cider, sugar, and maple syrup in a small saucepan. Stir over moderate heat for several minutes to melt the sugar. Do not boil. Pour the liquid into a heatproof bowl. Add the butter, vanilla, and salt. Stir, then set aside. In a large bowl, whisk about ½ cup of the hot sweeteners into the yolks to temper—or be warmed gradually—so that the yolks don’t cook and curdle. Continue, about ½ cup at a time, until all of the sweetener is added. Set aside. In another large bowl, using an electric mixer, beat the egg whites until they form soft, droopy peaks. Fold the whites into the cider mixture with a few strokes; there should be liquid under the whites. Pour the filling into the pie shell and smooth the top with a spoon. Bake on the center oven rack for about 40 minutes, until the filling is set under the soft topping. To check, poke the pie plate; the filling should not move in waves. The top of the pie will be dark. Transfer the pie to a rack and cool thoroughly. Serve at room temperature or chill first, covered with tented aluminum foil.
How to Make Boiled Cider
Pour 7 cups of fresh, preservative-free apple cider into a large stainless steel or enameled pot. Bring to a boil. Continue to boil gently, stirring often, until the cider is reduced to 1 cup of liquid. (Use a glass measuring cup to check.) Stir often.
This old-fashioned pie was popular with the Shakers, a religious sect that flourished in this country from the late 1700s until the mid-1900s. They have all but disappeared, but they left behind an enduring legacy that includes, among other things, a reputation of being creative inventors, farmers, and cooks. Maple syrup and boiled cider were two of their favorite sweeteners. If you prefer not to use commercial boiled cider, you can make your own (see “How to Make Boiled Cider,” below).–Courtesy of The Old Farmer's Almanac Everyday Baking.