Makes 16 large or 32 small cookies
Make ahead: The dough needs to be refrigerated for at least 1 hour and up to overnight. The caramel needs to set on the baked shortbread for at least 1 hour and preferably overnight. The finished shortbread can be stored between layers of wax paper in an airtight container for up to 5 days or frozen for up to 1 month.
For the shortbread:
16 tablespoons (2 sticks) cold unsalted butter, cut into small cubes
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½ cup superfine sugar
1½ cups plus 2 tablespoons flour
1 teaspoon vanilla extract
For the topping:
7 tablespoons (from 1 stick) unsalted butter
3 cup superfine sugar
1 tablespoon Lyle’s Golden Syrup (may substitute dark corn syrup)
1 cup plus 2 tablespoons homemade or store-bought dulce de leche (from one 13.4-ounce can)
8 teaspoon plus ¼ teaspoon fine sea salt
7 ounces bittersweet chocolate chips (60 to 70 percent cacao)
2 ounces white chocolate, broken into pieces
1. For the shortbread: Grease a 9-inch square baking pan with cooking oil spray, then line with parchment paper so the paper hangs over two sides (for easy lifting).
2. Combine the butter, sugar, flour and vanilla extract in a food processor. Pulse to form a dough that holds together. Transfer to the pan and press the dough into the bottom; feel free to lay a piece of plastic wrap over the dough for this step. Use a fork to prick the dough. Refrigerate for at least one hour and up to overnight.
3. Preheat the oven to 300 degrees. Bake the shortbread dough, straight from the refrigerator, for 40 to 45 minutes; it should just be starting to brown. Cool in the pan.
4. Meanwhile, make the topping: Combine the butter, sugar, syrup, dulce de leche and salt in a small nonstick saucepan over medium heat. stirring until the mixture is melted and blended. Reduce the heat to low; cook for eight or nine minutes, stirring often, to form a caramel; when it’s ready, it will be darkened in color with a thinner, pourable consistency.
5. Pour the caramel directly over the center of the shortbread base; it will spread by itself. You don’t want to work it into the corners. Leave it undisturbed for at least one hour to set.
6. Heat a few inches of water in a saucepan over medium heat. Place the bittersweet chocolate chips in a bowl that fits over the saucepan. Once the chocolate has melted and can be stirred smooth, pour it over the set caramel. Use a bamboo skewer or cake tester to create swirl patterns in the chocolate.
7. Repeat the melting and swirling with the white chocolate. If either chocolate begins to firm up prematurely, transfer to a low-temperature oven just until softened.
8. Cool until the chocolate is set yet slightly soft. Use the parchment paper to lift and transfer the shortbread to a cutting board. A serrated knife works best for cutting into 16 or 32 squares. Allow the chocolate to set completely before serving or storing.
Nutritional information per cookie (based on 32): 190 calories, 2 g protein, 21 g carbohydrates, 12 g fat, 8 g saturated fat, 25 mg cholesterol, 40 mg sodium, 0 g dietary fiber, 15 g sugar
Adapted by The Washington Post from “The Picnic Cookbook,” by Annie Bell (Kyle, 2013).