- 6 tablespoons cold unsalted butter, cut into 1/2-inch pieces, plus more, softened, for pan
- 3/4 cup all-purpose flour
- 3 tablespoons light-brown sugar
- Rounded 1/4 teaspoon coarse salt
- 1/2 cup finely shredded desiccated unsweetened coconut
- 3 ounces white chocolate, coarsely chopped (about 1/2 cup)
- 3 large eggs, room temperature
- 2/3 cup granulated sugar
- 3 tablespoons all-purpose flour
- 2/3 cup fresh Key lime juice or regular lime juice
- 1 tablespoon confectioners' sugar, for dusting
- Crust: Preheat oven to 400 degrees. Lightly butter an 8-inch square baking pan. Line bottom and 2 sides of pan with parchment, leaving a 1-inch overhang; lightly butter the parchment.
- Combine flour, brown sugar, and salt in a food processor, and pulse a few times to combine. Add butter, and pulse until largest pieces are the size of peas. Add coconut and white chocolate, and pulse a few more times, just until mixture holds together. Transfer mixture to pan, and pack down firmly with the bottom of a measuring cup. Bake until crust is deep golden brown, 18 to 20 minutes. Remove from oven, and reduce heat to 300 degrees.
- Filling: Whisk together eggs and granulated sugar in a bowl, then whisk in flour. Gently stir in lime juice (do not whisk or stir vigorously or batter will become foamy).
- Pour filling over hot crust, and bake until filling is set, about 15 minutes. Let cool completely on a wire rack.
- Run the tip of a knife along the 2 sides of the pan not lined with parchment, then remove from pan by lifting with parchment overhang. Transfer to a cutting board, dust with confectioners' sugar, and cut into 1-inch squares.
Key limes, so called because they were once cultivated in the Florida Keys, are also known as West Indian, Mexican, or bartender's limes. You'll need anywhere from one-half to two-thirds of a pound, depending on how fresh and juicy they are. If you can't find Key limes, regular limes also make delicious cookies.