• Center a rack in the oven and preheat it to 350 degrees F. Generously butter (or spray) an 8-inch square pan.

For the Shortbread Crust

• Whisk together the flour and cocoa powder.
• Working with a stand mixer fitted with the paddle attachment, or in a large bowl with a hand mixer, beat the butter, sugar and salt together on medium speed until smooth, about 2 minutes. Turn off the mixer, add the dry ingredients all at once and beat on low until the dough comes together. This will take a little longer than you might expect, so don’t get discouraged. Give the dough a few last turns with a flexible spatula and scrape it out into the pan. Using your fingertips, pat the dough evenly over the bottom of the pan; press down and make sure to get the dough into the corners.
• Bake the shortbread for 21 to 23 minutes, rotating the pan after 10 minutes, or until it is slightly darker around the edges and just starting to pull away from the sides of the pan; it will not feel completely firm if poked gently. Transfer the pan to a rack and allow the shortbread to cool completely.

For the Caramel Topping

• Put a heatproof bowl close to the stovetop, along with a small bowl of cold water and a pastry brush (preferably silicone).
• Put the sugar, water and corn syrup in a small saucepan or skillet and bring to a boil over medium-high heat. Allow the ingredients to bubble away, brushing down the sides of the pan with cold water if there are spatters and swirling the pan a couple of times once you start seeing some color, until the caramel turns a pale to medium amber. (Don’t let it get as dark as mahogany.) Lower the heat, stand away from the pan and add the cream, followed by the butter and salt — ingredients that might cause the caramel to roil. Remove the pan from the heat, drop in the chocolate and, using a silicone spatula or wooden spoon, stir to blend.
• Pour the hot caramel over the shortbread and sprinkle with the nuts. Allow to set at room temperature.
• When the caramel is firm, run a table knife around the sides of the pan, care- fully unmold the cookie onto a rack and turn it over onto a cutting board. Using a long thin knife, cut 21 bars (cut into 3 strips and then cut each strip into 7 bars). Alternatively, leave the cookie whole and cut individual bars as needed.
• To store: when the caramel is fully set, the cookies (cut or intact) can be wrapped in plastic and refrigerated for up to 5 days. Remove them from the refrigerator about 10 minutes before serving.