Nutrition per serving may change if servings are adjusted.
Key Lime Filling
¾ cup sugar
1 large egg
1 large egg white
¼ cup fresh or bottled Key lime juice
2 teaspoons freshly grated lime zest
¾ cup water, divided
2 tablespoons cornstarch
⅓ cup low-fat milk
¾ cup plus 2 tablespoons unsifted cake flour
1 teaspoon baking powder
¼ teaspoon freshly grated nutmeg
2 large eggs, at room temperature (see Tip)
2 large egg whites, at room temperature
1 cup sugar
2 teaspoons fresh or bottled Key lime juice
2 teaspoons vanilla extract
¾ cup sugar
3 large egg whites, at room temperature
¼ teaspoon cream of tartar
3 tablespoons water
1 teaspoon vanilla extract
Fresh lime slices, for garnish
To make Key lime filling: Whisk ¾ cup sugar, 1 egg and 1 egg white in a small saucepan until well combined. Whisk in ¼ cup lime juice, lime zest and ½ cup water. Stir over low heat until the sugar dissolves. Remove the pan from the heat.
Stir together cornstarch and remaining ¼ cup water in a small bowl; whisk into the egg mixture. Return to the heat, increase the heat to medium-high and continue whisking until the filling comes to a boil and thickens. Whisk for 30 seconds longer and remove from the heat. Transfer the filling to a small bowl and place a piece of wax paper or plastic wrap directly on the surface to prevent a skin from forming. Refrigerate until cool, about 1 hour.
To make cake: Preheat oven to 350°F. Line the bottom of a 9-inch round cake pan with parchment or wax paper. Coat the paper and inside of the pan with cooking spray.
Heat milk in a small pan until steaming; remove from the heat, cover and set aside. Sift flour, baking powder and nutmeg into a bowl; set aside.
Beat 2 eggs and 2 egg whites in a mixing bowl with an electric mixer on low speed until frothy. Add 1 cup sugar and beat on medium speed until the eggs are very fluffy and pale, a full 3½ minutes. Add 2 teaspoons lime juice, vanilla and the reserved dry ingredients. Beat on low speed just until the flour is incorporated. Beat in the hot milk (the batter will be thin).
Pour the batter into the prepared pan. Bake until the top feels firm when touched lightly and a skewer inserted in the center of the cake comes out clean, 23 to 25 minutes. Let the cake cool in the pan for 5 minutes. Run a knife around the inside of the pan to loosen the edges. Turn the cake out of the pan and set it right-side up on a wire rack to cool completely.
To fill cake & make meringue: To cut a depression in the cake for the filling, hold scissors with the tips angled slightly down toward the center of the cake and snip a ¼-inch-deep circle around the cake, ½ inch in from the cake's rim. Cut horizontally to the center of the cake with a serrated knife, angling down toward the center to create the depression but leaving the bottom of the cake intact. Lift off the cake "lid." Set the cake bottom on a large ovenproof plate or baking sheet. Spread the chilled lime filling in the depression and replace the cake lid on top.
Preheat oven to 375°F. Bring about 1 inch of water to a simmer in a large saucepan. Put ¾ cup sugar, 3 egg whites, cream of tartar and 3 tablespoons water in a metal bowl that will fit over the saucepan. Set the bowl over the gently simmering water and beat with an electric mixer on low speed, moving the beaters around the bowl constantly, until an instant-read thermometer registers 140°F, 3 to 5 minutes. Increase the mixer speed to high and continue beating over the heat for 3½ minutes. Remove the bowl from the heat and beat until the meringue is cool, about 5 minutes. Beat in vanilla.
Spread the meringue over the sides and top of the filled cake with a spatula, mounding it toward the center and swirling it in peaks. Bake until the meringue is golden, about 10 minutes.
Place the cake on a wire rack and let cool to room temperature, about 1 hour. If the cake is on a baking sheet, transfer it to a serving plate. Serve immediately, or cover carefully with plastic wrap that has been lightly sprayed with cooking spray and refrigerate for up to 8 hours. Just before serving, garnish with lime slices.
Make Ahead Tip: Cover the cake carefully with plastic wrap that has been lightly coated with cooking spray and refrigerate for up to 8 hours.
Tip: To bring an egg to room temperature: Either set it out on the counter for 15 minutes or submerge it (in the shell) in a bowl of lukewarm (not hot) water for 5 minutes.