If you cannot find fresh Key limes, use regular (Persian) limes. Do not use bottled lime juice. Grate the zest from the limes before juicing them, avoiding the bitter white pith that lies just beneath the outermost skin. The optional coconut garnish adds textural interest and tames the lime flavor for those who find it too intense. The recipe can be doubled and baked in a 13 by 9-inch baking pan; you will need a double layer of extra-wide foil for the pan (each sheet about 20 inches in length) and should increase the baking times by a minute or two.
5 ounces animal crackers
3 tablespoons packed brown sugar (light or dark)
4 tablespoons unsalted butter, melted and cooled slightly
2 ounces cream cheese, at room temperature
1 tablespoon grated lime zest, minced
1 (14-ounce) can sweetened condensed milk
1 egg yolk
1/2 cup fresh lime juice, from about 20 Key limes or about 3 Persian limes
3/4 cup sweetened shredded coconut, toasted until golden and crisp
1. Adjust an oven rack to the middle position and heat the oven to 325 degrees. Cut about a 12-inch length of extra-wide heavy-duty foil; fold the cut edges back to form a 71/2-inch width. With the folded sides facing down, fit the foil securely into the bottom and up the sides of an 8-inch-square baking pan, allowing the excess to overhang the pan sides. Spray the foil with nonstick cooking spray.
2. To make the crust: In the workbowl of a food processor, pulse the animal crackers until broken down, about ten 1-second pulses; process the crumbs until evenly fine, about 10 seconds (you should have about 11/4 cups crumbs). Add the brown sugar and salt; process to combine, ten to twelve 1-second pulses (if large sugar lumps remain, break them apart with your fingers). Drizzle the butter over the crumbs and pulse until the crumbs are evenly moistened with the butter, about ten 1-second pulses. Press the crumbs evenly and firmly into the bottom of the prepared pan. Bake until deep golden brown, 18 to 20 minutes. Cool on a wire rack while making the filling. Do not turn off the oven.
3. To make the filling: While the crust cools, in medium bowl, stir the cream cheese, zest, and salt with a rubber spatula until softened, creamy, and thoroughly combined. Add the sweetened condensed milk and whisk vigorously until incorporated and no lumps of cream cheese remain; whisk in the egg yolk. Add the lime juice and whisk gently until incorporated (the mixture will thicken slightly).
4. To assemble and bake: Pour the filling into the crust; spread to the corners and smooth the surface with a rubber spatula. Bake until set and the edges begin to pull away slightly from the sides, 15 to 20 minutes. Cool on a wire rack to room temperature, 1 to 11/2 hours. Cover with foil and refrigerate until thoroughly chilled, at least 2 hours.
5. Loosen the edges with a paring knife and lift the bars from the baking pan using the foil extensions; cut the bars into 16 squares. Sprinkle with the toasted coconut, if using, and serve. (Leftovers can be refrigerated up to 2 days; the crust will soften slightly. Let the bars stand at room temperature about 15 minutes before serving.)
Variation Triple Citrus Bars
Using three types of citrus (orange, lemon, and lime) gives these bars a slightly more complex, floral flavor.
Follow the recipe for Key Lime Bars, substituting 11/2 teaspoons each grated lime zest, lemon zest, and orange zest (mince zest after measuring) for the lime zest, and using 6 tablespoons lime juice, 1 tablespoon lemon juice, and 1 tablespoon orange juice in place of all lime juice.