For the brownie layer:
6 Ounces bittersweet chocolate, coarsely chopped
3 Ounces unsweetened chocolate, coarsely chopped
2 Sticks (8 ounces) unsalted butter, cut into chunks
1 2/3 Cups sugar
4 Large eggs
1/2 tsp. salt
1/2 tsp. pure vanilla extract
1 Cup all-purpose flour
1 Cup walnuts, coarsely chopped (or any nuts of your choice)
For the cookie layer:
1 1/4 Cups all-purpose flour
1/2 tsp. baking soda
1/2 tsp. salt
1 1/2 sticks (12 tablespoons) unsalted butter, at room temperature
3/4 Cup (packed) light brown sugar
2/3 Cup sugar
1 Large egg
1 Egg yolk
1 tsp. pure vanilla extract
1 Cup store-bought chocolate chips
*I sprinkled an extra cup of chocolate chips on top of the cookie layer*
Center a rack in the oven and preheat the oven to 350° F.
Butter a 9-x-13-inch baking pan, line it with wax or parchment paper and butter the paper. Put the pan on a baking sheet.
To make the brownie batter:
Put both chocolates and the butter in a bowl set over a saucepan of simmering water. Stirring occasionally, heat just until the ingredients are melted, shiny and smooth.
If the mixture gets too hot, the butter will separate from the chocolates. Remove the bowl from the heat.
Working with a stand mixer, preferably fitted with a paddle attachment, or with a hand mixer in a large bowl, beat the sugar and eggs on medium-high speed for about 2 minutes, until pale, thick and creamy. Beat in the salt and vanilla extract.
Reduce the speed to low and mix in the melted chocolate and butter, mixing only until incorporated.
Scrape down the sides of the bowl with a rubber spatula, then, still on low speed, add the flour, mixing only until it disappears into the batter.
Using the spatula, fold in the walnuts, and scrape the batter into the prepared pan. Set aside.
To make the cookie dough:
Whisk together the flour, baking soda, and salt.
Working with a stand mixer in the cleaned bowl or with the hand mixer in another large bowl, beat the butter and both sugars together on medium-high speed until smooth and creamy, about 3 minutes.
One at a time, add the egg and the yolk, beating for 1 minute after each addition.
Beat in the vanilla. Reduce the mixer speed to low and add the dry ingredients, mixing only until they disappear into the dough. Still on low, mix in the chopped chocolate.
Drop the cookie dough by spoonfuls over the brownie batter an, using a spatula and a light touch, spread it evenly over the batter.
Bake for 50 to 55 minutes, or until the cookie top is deep golden brown and firm and a thin knife inserted into the brownie layer comes out with only faint streaks of moist chocolate. Transfer the pan to a rack and cool to room temperature.
When the brownies are completely cool, carefully run a knife between the sides of the pan and the brownies, then invert them onto another rack, remove the paper and turn right side up onto a cutting board.
Cut into bars.