1 1/2 sticks unsalted butter, cut into 1/2-inch pieces
2 cups all-purpose flour
1 cup confectioner’s sugar (or 1/2 cup packed light brown sugar)
1/2 teaspoon salt
For the custard:
4 large eggs
1 1/2 cups granulated sugar
3/4 cup key lime juice
1/3 cup all-purpose flour
3 tablespoons confectioners' sugar
How to make it
Preheat the oven to 350°F.
In a food processor process the butter, confectioner's sugar, flour and salt until the mixture resembles coarse breadcrumbs and can hold together when pressed up against the side of the food processor bowl.
Sprinkle the crumb mixture into a 13 x 9 x 2-inch baking pan and press down onto the bottom of the pan as evenly as possible. If necessary, flatten it with the flat blade of a sturdy spatula, smoothing it out.
Bake the shortbread crust about 20 minutes, until it starts to turn golden.
While the shortbread is baking, prepare the custard.
In a bowl, whisk together the eggs and granulated sugar. Stir in key lime juice and flour, mixing very well with a whisk.
Pour the custard mixture over the hot shortbread.
Reduce the oven temperature to 300°F and bake until it is set, about 30 minutes.
Let it cool completely in the pan, then sift some confectioner sugar over bars before cutting and serving.