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