Key Lime Bars

1. Adjust oven rack to middle position and heat oven to 325 degrees. Make foil sling by folding 2 long sheets of aluminum foil so that they are as wide as 8-inch baking pan. Lay sheets of foil in pan perpendicular to one another, with extra foil hanging over edges of pan. Push foil into corners and up sides of pan, smoothing foil flush to pan. Grease foil.
2. FOR THE CRUST: Pulse animal crackers in food processor until broken down, about 10 pulses; process crumbs until evenly fine, about 10 seconds (you should have about 1-1⁄4 cups crumbs). Add brown sugar and salt; process to combine, 10 to 12 pulses (if large sugar lumps remain, break them apart with your fingers). Drizzle butter over crumbs and pulse until crumbs are evenly moistened with butter, about 10 pulses. Press crumbs evenly and firmly into bottom of prepared pan. Bake until deep golden brown, 18 to 20 minutes. Cool on wire rack while making filling. Do not turn off oven.
3. FOR THE FILLING: While the crust cools, in medium bowl, stir the cream cheese, zest, and salt with rubber spatula until softened, creamy, and thoroughly combined. Add sweetened condensed milk and whisk vigorously until incorporated and no lumps of cream cheese remain; whisk in egg yolk. Add lime juice and whisk gently until incorporated (mixture will thicken slightly).
4. Pour filling into crust; spread to corners and smooth surface with rubber spatula. Bake until set and edges begin to pull away slightly from sides, 15 to 20 minutes. Let cool on wire rack to room temperature, 1 to 1-1⁄2 hours. Cover with foil and refrigerate until thoroughly chilled, at least 2 hours.
5. Remove bars from pan using foil; cut bars into 16 squares. Sprinkle with toasted coconut, if using, and serve. (Bars can be refrigerated for up to 2 days; crust will soften slightly.)

TRIPLE CITRUS BARS
Using three types of citrus (orange, lemon, and lime) gives these bars a slightly more complex, floral flavor.
Substitute 1-1⁄2 teaspoons each grated lime zest, lemon zest, and orange zest for lime zest, and use 6 tablespoons lime juice, 1 tablespoon lemon juice, and 1 tablespoon orange juice in place of all lime juice.