2 large egg whites, at room temperature, or equivalent dried egg whites, reconstituted according to package directions (see Ingredient notes)
¼ cup sugar
1To prepare cake: Preheat oven to 350°F. Coat two 9-inch round cake pans with cooking spray with flour (see Tip); alternatively, coat the pans with regular cooking spray, line them with parchment paper and spray the paper.
2Whisk all-purpose flour, whole-wheat flour and salt in a medium bowl.
3Beat 4 eggs and ⅔ cup sugar in a large bowl with an electric mixer on medium-high speed until pale yellow and doubled in volume, 5 to 10 minutes. Beat in vanilla and 1 teaspoon lime zest.
4With a rubber spatula, gently fold almond (or canola) oil into the egg mixture, using long, even strokes, until just incorporated. Add the flour mixture; gently stir until just incorporated. Divide the batter between the pans; spread to the edges and gently rap the pans against the counter once or twice to settle the batter.
5Bake, rotating the pans halfway through, until lightly browned and a toothpick inserted into the center comes out clean, 20 to 22 minutes. Cool on a wire rack for 10 minutes, then invert the layers onto the rack, remove the pans and parchment paper, if using, and let cool completely, about 45 minutes.
6To prepare lime filling: Whisk condensed milk, lime juice and 1 teaspoon lime zest in a medium bowl until combined. Refrigerate until thickened, at least 30 minutes or overnight.
7To prepare meringue: Beat egg whites in a medium bowl with an electric mixer at high speed until soft peaks form. Beat in ¼ cup sugar in a slow, steady stream. Continue beating until stiff peaks form.
8Position oven rack about 6 inches from the heat source; preheat broiler. Transfer one layer, bottom-side down, to a large baking sheet. Spread the meringue on top of the cake, creating decorative peaks and valleys in the meringue with a rubber spatula. Broil until lightly browned, watching carefully to prevent burning, 30 seconds to 1 minute.
9To assemble cake: Place the plain layer, top-side down, on a serving plate; spread the lime filling on top. Cover with the meringue-topped layer.
Make Ahead Tip: Wrap the cake layers in plastic wrap and store at room temperature for up to 1 day. Cover and refrigerate the lime filling for up to 1 day. Prepare meringue and assemble the cake just before serving.
When using cake pans, they must be greased and floured to create a thin layer of protection against the oven's heat. For greater convenience, use a cooking spray that has flour in the mix, such as Pam for Baking, Baker's Joy or Crisco No-Stick Flour Spray.
Whole-wheat pastry flour has less gluten-forming potential than regular whole-wheat flour, making it a better choice for tender baked goods.
To properly measure flour when baking, use a spoon to lightly scoop flour from its container into a measuring cup. Once in the measuring cup, use a knife or other straight edge to level the flour with the top of the cup. If the measuring cup is dipped directly into the container—a common mistake—the flour will be packed into the cup and result in extra flour being added to the recipe, yielding tough, dense baked goods.
Room-temperature butter for a batter is one of the biggest culinary missteps. In fact, butter must be below 68°F to trap air molecules and build structure. Otherwise, the fat will be liquefied and the cake will be flat. To get “cool” butter: Cut refrigerated butter into chunks and let them sit in a bowl for 5 minutes before beating.
Eggs must be at room temperature for the proteins to unwind enough to support the cake's crumb. Either set the eggs out on the counter for 15 minutes or submerge them in their shells in a bowl of lukewarm (not hot) water for 5 minutes.
Although you cannot overbeat the eggs, sugar and butter, you can overbeat the flour. If you do, you'll develop the gluten and create a quick bread rather than a layer cake. Beat the flour just until there are no white grains of undissolved flour visible but not until the batter is smooth.
Ingredient notes: Key limes are more fragrant, acidic and juicy than the common Persian lime found in U.S. markets. Look for the small, round limes with yellow-green skin in the specialty-produce section of supermarkets or Latin food markets.
Dried egg whites are pasteurized—a wise choice when making an uncooked meringue. They are also convenient when you have no use for leftover yolks. You'll find them in the baking or natural-foods section of most supermarkets.