The Neiman Marcus Cookie

INGREDIENTS

• 2½ cups rolled oats
• 2 cups all-purpose flour
• 1 teaspoon baking powder
• 1 teaspoon baking soda
• ½ teaspoon salt
• 1 cup (2 sticks) unsalted butter, at room temperature
• 1 cup granulated sugar
• 1 cup brown sugar
• 2 eggs
• 1 teaspoon vanilla extract
• 12 ounces semisweet chocolate chips
• 4 ounces milk chocolate chips
• 1½ cups chopped walnuts