One of my favorite things about shopping at farmers' markets is the remarkable diversity of the fruits and vegetables on display. Even if I go to the market in search of some seemingly straightforward ingredient, such as a lime, I'm always presented with several options to choose from. A sign marking "Rangpur limes" at one stand will instantly draw me in. Are those Kaffir limes at another stand? Well, I guess I'll just have to try both! Unusual, rare varieties--the stranger the better--are one of my weaknesses. For a long while, I thought a lime was just a lime and that was that, but there are actually several different types, each with a unique aroma, appearance, and flavor.
The most common lime found in the United States is the Persian lime, which is sometimes called Tahiti or Bearss lime. It's juicy and conveniently seedless, thanks to a genetic peculiarity (the Persian lime has three sets of chromosomes rather than the usual two). In other parts of the world, the standard sour citrus is the Key lime, also known as the Mexican, true, or bartender's lime. This lime is smaller, tarter, and more aromatic than the Persian lime, and it also has a longer history. Ancient Arabs likely brought Key limes from Southeastern Asia to the Middle East, Crusaders carried them west to the Mediterranean, and Spaniards introduced them to the Caribbean. Eventually, they were grown throughout the Florida Keys and soon became known as a necessary ingredient for the eponymous pie.
Another variety, thought to be a natural hybrid of citron and either sweet or sour orange, is called the sweet lime. As their name suggests, sweet limes are mild and barely acidic, so are often pressed and served as juice. Rangpur limes look more like mandarins. They're relatively large with orange skin and flesh, yet they have a distinct lime flavor--tart and rich with pine and floral notes. Kaffir limes, native to Southeast Asia, are bumpy and contain only a squirt of juice. But in this case, the fruit isn't necessarily the prized part of the tree. Aromatic Kaffir leaves are more commonly used for cooking, especially in Thai curries. Have you ever seen a finger lime? It's a curious fruit, shaped like an elongated olive and filled with pink, caviar-like bubbles of tart juice. Then there's the limequat, a 100-year old variety that can be eaten whole, like a kumquat, or can be zested and juiced, like a lime.
Still other types of lime are out there, waiting to be discovered. Head to the farmers' market and find out which one is your favorite.
This isn't the easiest cake to throw together, but the cake layers and filling can be made ahead of time. The extra effort is well worth it -- dig in and celebrate!
1 ½ cups all-purpose flour
2 teaspoons baking powder
¾ teaspoon kosher salt
1 tablespoon lemon zest
2 teaspoons orange zest
1 cup granulated sugar
1 cup plain whole milk yogurt
3 large eggs
1 teaspoon orange liquor
1/3 cup neutral-tasting oil, such as safflower oil
2 tablespoons freshly squeezed lemon juice
2 large eggs
3 large egg yolks
6 tablespoons unsalted butter, cut into 6 pieces
1/3 cup granulated sugar
½ teaspoon kosher salt
3 large egg whites
¾ cup granulated sugar
1 tablespoon agave
¼ cup tequila
For the cake layers: Preheat the oven to 350°F. Generously butter and flour the inside surfaces of an 8 ½ by 4 ½-inch loaf pan.
In a small bowl, whisk together the flour, baking powder, and salt.
In a large bowl, combine the lemon zest, orange zest, and sugar. Using your fingers, rub the zest into the sugar for a minute, or until it smells very fragrant. Add the yogurt and eggs. Whisk vigorously until the mixture is smooth. Dump in the flour mixture, then whisk until there are no visible lumps. Whisk in the orange liquor. Using a rubber spatula, fold in the oil. When the oil is fully incorporated, the cake batter will have an even, gorgeous sheen. Scrape the batter into the prepared loaf pan. Hold the spatula vertically and drag it lengthwise through the top inch of batter. This subtle gesture creates a seam for the cake to expand. Bake until a skewer or toothpick inserted in the center comes out clean, about 55 to 60 minutes. Let the cake cool to room temperature. (Cake can be made 1 day ahead, cooled, and wrapped tightly in plastic wrap).
For the lime curd: Zest the limes, reserving the zest in a small bowl, then juice the limes. There should be about ¼ cup of lime juice. In a heatproof bowl set over a pot of simmering water, whisk the lime juice, lemon juice, eggs, yolks, butter, sugar, and salt. Cook, stirring constantly, until the curd thickens and coats the back of a spoon, about 7 minutes. Pass the curd through a fine-mesh strainer, then stir in the reserved lime zest and let cool to room temperature. (Lime curd will keep, refrigerated, in a tightly sealed container for one week.)
For the tequila frosting: In the heatproof bowl of a stand mixer set over a pot of simmering water, whisk the egg whites, sugar, agave, and tequila. Continue whisking until the sugar dissolves and the mixture registers 160°F on an instant-read thermometer, about 3 minutes. Immediately transfer the bowl to the stand mixer and whisk on high speed until the frosting is thick and glossy, about 5 - 7 minutes. (Tequila frosting is best used straight away.)
To assemble the margarita cake: Using a long, serrated knife held parallel to the cutting board, slice the cake lengthwise into three even layers. Place the bottom layer on a serving platter. Spread about ½ cup of lime curd over the cake, then top it with the second cake layer. Spread another ½ cup of lime curd, then place the final cake layer on top. (You may have some lime curd left over, but you'll have no trouble using it up. Try swirling a spoonful into plain yogurt.) Spread tequila frosting evenly over the top and sides of the cake. Serve.