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 teaspoon salt
1/2 teaspoon vanilla extract
1 cup all-purpose flour
For the cookie layer
1 1/4 cups all-purpose flour
1/2 teaspoon baking soda
1/2 teaspoon salt
1 1/2 sticks (12 tbsp) unsalted butter, at room temperature
3/4 cups (packed) light brown sugar
2/3 cup sugar
1 large egg
1 large egg yolk
1 teaspoon pure vanilla extract
1 cup chocolate chips
To make the brownie batter, melt both chocolates and the butter together in a bowl set over a saucepan of simmering water, stirring occasionally. Melt and stir just until the mixture is shiny and smooth, then remove the bowl from the heat and set aside.
Meanwhile, beat the sugar and eggs together with a mixer on medium high speed for about 2 minutes, until pale, thick and creamy. Beat in the salt and vanilla, and then reduce the speed to low and mix in the melted chocolate and butter, mixing just until incorporated. Scrape down the sides of the bowl and then add the flour, mixing on low speed just until it disappears into the batter. Scrape the batter out into a greased 9x13 pan. Set aside.
To make the cookie dough, first whisk together the flour, baking soda and salt in a small bowl, and set aside. 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 egg yolk, beating for 1 minute after each addition. Beat in the vanilla, and then reduce the speed to low and add the dry ingredients, mixing only until they disappear into the batter. Still on low, mix in the chopped chocolate.
Drop the cookie dough by spoonfuls onto the brownie batter, and then use a spatula to gently smooth out the cookie dough layer evenly over the batter.
Bake for 50 to 55 minutes at 350 degrees, until the cookie top is deep golden brown and a sharp knife inserted into the pan comes out with only faint streaks of moist chocolate.
For the best-shaped brownies, practice your patience and wait until they’re cool to cut them. (I was in a hurry and put them in my freezer for a while.)