For the Shortbread:

1. PREP: Heat oven to 350 degrees. Grease a 9-by-13-inch baking dish using butter or baking spray, and then line with parchment paper so that there is a 2-inch overhang on the two long sides of the dish.
2. MAKE THE SHORTBREAD: Using a stand mixer or a hand-held electric mixer fitted with a paddle attachment (or beaters), mix together the flour, sugar, salt, and butter at low speed until the dough comes together but is still a little crumbly. Be patient. Since the butter is cold, it will take a few minutes for the dough to start to come together. (You can also pulse together ingredients in a food processor).
3. BAKE: Dump the dough into the prepared pan, spread the mixture evenly, and then press it into the pan using your hand or the bottom of a measuring cup or glass. Next, prick the dough all over with the tines of a fork. Bake the shortbread until golden, about 30 to 35 minutes. Remove pan from the oven and set aside to cool slightly.
4. Decrease the oven temperature to 325 degrees.
5. MAKE THE BROWNIES: While the shortbread is baking, make the brownies. Over low heat, melt the butter in a saucepan with the cocoa and stir until smooth. Don’t boil. You just want to melt the butter and not cook it. Remove from heat and allow to cool for a few minutes, then transfer to a large bowl.
6. ADD THE EGGS: Whisk in eggs, one at a time. Then, stir in vanilla. One note: make sure the butter and cocoa mixture is cool before adding the eggs! You don’t want to cook the eggs. 
7. ADD DRY INGREDIENTS: In a separate bowl, combine sugar, flour, salt, and nuts (if using.) Add to the cocoa-butter mixture and stir until just combined.
8. BAKE: Pour the batter on top of the warm shortbread and bake for 35 to 40 minutes. The brownie top should be set and the edges start pulling away from the pan. But the centers will still be soft and gooey.
9. COOL AND SERVE: Transfer to a wire rack to cool completely and sprinkle with flaky sea salt if using. Cut into bars before serving.