Directions

• Step 1 Preheat oven to 350 degrees F. Prepare a 9-inch by 13-inch baking dish by greasing it and lining it with parchment paper. Cut the parchment so that it hangs over the long sides of the baking dish by about 2 inches on each long side. Use these long parchment overhangs as handles to help you remove the baked shortbread from the pan without breaking it.
• Step 2 To prepare the shortbread, put flour, sugar and salt into the bowl of an electric stand mixer fitted with the paddle attachment. Mix briefly to combine. Turn the mixer to a low speed setting, add the butter and mix just until you have a crumbly mixture. Press this crumbly mixture into the prepared pan. Prick the shortbread all over with the tines of a fork. Bake the shortbread until it is firm and golden brown. This will take between 30 and 35 minutes. Remove the shortbread from the oven and raise the oven temperature to 375 degrees F. for the next step.
• Step 3 While the shortbread is baking, prepare the brownie batter. Melt the butter and chocolate together. You can do this by putting the butter and chocolate in a large metal bowl, setting it over a pan of simmering water, and stirring the mixture until it is melted and smooth. Alternatively, you can melt the butter and chocolate in the microwave in 30 second bursts–stirring after each burst until you have a smooth melted butter/chocolate mixture. Remove the melted chocolate/butter mixture from the heat and whisk the sugars and the cocoa powder into the hot mixture. The addition of the sugars and cocoa powder at this point will cool down the batter so that you can whisk in the eggs and the vanilla.
• Step 4 Put the flour and salt into a medium bowl and whisk them together. Whisk this mixture into the chocolate mixture. Continue to whisk until no flour streaks remain in the batter. Fold in the nuts at this point if you are using them. Spread this mixture on top of the warm shortbread that you have just baked. Sprinkle the brownie batter with the flaky sea salt.
• Step 5 Bake the brownie shortbread for 23 to 28 minutes. Your brownie shortbread will be done when the top of the brownie is set. Note that a toothpick inserted into the middle of your brownie shortbread will still come out gooey when the top is set. (I baked my brownie shortbread for 23 minutes.) Transfer the brownie shortbread to a wire rack to cool completely before cutting into bars and serving.