Inspired by Cooks Illustrated Key Lime Bars
5 ounces animal crackers (about 1 1/4 C crumbs)
3 T light or dark brown sugar packed
pinch of salt
4 tbsp butter melted and cooled slightly
1 1/2 cups rhubarb, chopped roughly
1 cup granulated sugar
1 1/2 cups water
2 ounces cream cheese, room temperature
1/2 cup sweetened condensed milk
1 large egg yolk
1 T fresh squeeze lime juice
1. Adjust oven rack to middle position and heat oven to 325 degrees. Cut about 12-inch length extra-wide heavy duty foil; fold cut edges back to form 7 1/2-inch width. With folded sides facing down, fit foil securely into bottom and up sides of 8-inch square baking pan, allowing excess to overhang pan sides. Spray foil with non-stick cooking spray.
2. To make rhubarb pulp: in a medium sauce pan, bring rhubarb, granulated sugar and water to a boil. Reduce heat and simmer until the rhubarb begins to break down, about 10 minutes. Remove from heat until cool enough to handle. Push the mixture through a mesh sieve, reserving the liquid (rhubarb simple syrup) for other tasty uses. Allow rhubarb pulp to cool completely.
3. To Make Crust. In work bowl of food processor, pulse animal crackers until broken down, about ten 1-second pulses; process crumbs until evenly fine, about 10 seconds. Add brown sugar and salt; process to combine, ten to twelve 1-second pulses. Drizzle butter over crumbs and pulse until crumbs are evenly moistened with butter, about ten 1-second 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.
4. To Make Filling: While crust cools, in medium bowl, stir 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 rhubarb pulp and lime juice and whisk gently until incorporated (mixture will thicken slightly).
5. To Assemble and Bake: 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. 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.
6. Loosen edges with pairing knife and lift bars from baking pan using foil extensions; cut bars into 16 squares. Sprinkle with toasted coconut if using, and serve. (Leftovers can be refrigerated up to two days; crust will soften slightly. Let stand at room temperature, about 15 minutes before serving.)