Whisk together the flour, baking powder and salt in a small bowl.
Melt the chocolate chips and butter in a medium microwave-safe bowl in the microwave in 30-second increments, stirring in between, until smooth. Let cool slightly.
Beat the sugar and eggs with an electric mixer on medium-high speed in a large bowl until thick and pale yellow, about 2 minutes. Scrape the chocolate mixture into the egg mixture and beat until incorporated. Beat in the vanilla. Add the flour mixture and beat on medium-low speed. Fold in the walnuts if using. The batter will be quite soft at this point. Refrigerate until firm enough to scoop, about 30 minutes.
Position oven racks in the top and bottom thirds of the oven and preheat to 175°C. Line 2 baking sheets with parchment paper.
Scoop heaping tablespoons of batter (a number-30 ice cream scoop works great, too) onto the prepared baking sheets about 2 inches apart (the cookies will spread considerably). (You will have additional batter for a second batch.)
Bake until the tops of the cookies are crackled and no longer glossy and the edges are firm, 14 to 16 minutes, rotating the sheets about halfway through. Let the cookies cool for a few minutes on the baking sheets, then transfer them to a wire rack to cool completely. When the baking sheets are cool, repeat with the remaining batter. The cookies can be stored in an airtight container at room temperature for up to 3 days.
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Cook's Note: When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
From Food Network Kitchen