1½ cups cold Unsalted Butter (3 sticks), cut into 1/2-inch pieces, plus more for greasing the pan
3 cups All-Purpose Flour
¾ cup granulated Sugar
1 ¼ teaspoons fine Sea Salt
1 cup plus 2 tablespoons Unsalted Butter , (2 1/4 sticks)
3 ounces Unsweetened Chocolate, chopped
1 ¼ cups Light Brown Sugar
1 cup Granulated Sugar
½ cup plus 1 tablespoon Cocoa Powder
3 large Eggs
1 tablespoon Vanilla Extract
1 ½ cups all-purpose Flour
¼ teaspoon fine Sea Salt
¾ cup slivered Almonds, Chopped Walnuts or Pecans, (optional)
Flaky Sea Salt
Heat oven to 350 degrees. Grease a 9-by-13-inch baking dish, and line with parchment paper so that there is a 2-inch overhang on the two long sides.
Prepare the shortbread: In the bowl of an electric mixer fitted with a paddle attachment or beaters, mix together flour, sugar and salt. Beat in butter on low speed until dough just comes together but is still a little crumbly. (Or pulse together ingredients in a food processor.)
Press dough into prepared pan. Prick dough all over with a fork. Bake until golden, 30 to 35 minutes. Remove from oven. Raise oven temperature to 375 degrees.
As shortbread bakes, prepare the brownie: Place butter and chocolate in a large bowl. Melt in the microwave in 30-second bursts, stirring after each burst, until smooth. (Alternatively, place bowl over a pot of simmering water, and heat chocolate and butter, stirring until smooth and melted.) Whisk in sugars and cocoa powder until smooth, then whisk in eggs and vanilla.
In a medium bowl, whisk together flour and sea salt. Whisk into chocolate mixture until no streaks of flour remain. Fold in nuts, if using. Spread mixture onto the warm shortbread base. Sprinkle lightly but evenly with flaky sea salt.
Bake until the top is set, the center is soft, and the edges start pulling away from the pan, 23 to 28 minutes. (A toothpick inserted into the center will come out gooey.) Transfer to a wire rack to cool completely. Cut into bars before serving.