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1
Center a rack in the oven and preheat the oven to 350 degrees F. Spray a 9x13-inch baking pan generously with cooking spray.
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2
Brownie batter:
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3
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. Remove the bowl from the heat. If you are more of a microwave person, you can microwave the chocolates and butter together in 30-second increments, stirring well after each one.
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4
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.
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5
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 (if using), and scrape the batter into the prepared pan. Set aside.
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6
Cookie dough:
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7
Whisk together the flour, baking soda, and salt. Working with the stand mixer in a 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 chocolate chips and walnuts (if using).
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8
Drop the cookie dough by spoonfuls over the brownie batter and, using a spatula and a light touch, spread it evenly over the batter.
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9
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 a few streaks of chocolate. Transfer the pan to a rack and cool to room temperature and serve!
