Chocolate Chip Cookie-Topped Brownies

INGREDIENTS

• :
• 6 ounces bittersweet chocolate
• 3 ounces unsweetened chocolate, coarsely chopped
• 2 sticks (8 ounces) unsalted butter, cut into chunks
• 1 2/3 cups sugar
• 4 large eggs
• ½ teaspoon salt
• ½ teaspoon pure vanilla extract
• 1 cup all-purpose flour
• 1 cup walnuts, coarsely chopped
• 1¼ cups all-purpose flour
• ½ teaspoon baking soda
• 1½ sticks (12 tablespoons) unsalted butter, at room temperature
• ¾ cup (packed) light brown sugar
• 2/3 cup sugar
• 1 large egg
• 1 large egg yolk
• 1 teaspoon pure vanilla extract
• 1 cup semisweet chocolate chips