How to Make It
Preheat oven to 350°F. Line 2 baking sheets with parchment paper. Beat brownie mix, melted butter, and egg together with an electric mixer on medium speed until just combined, about 1 minute. Stir in chocolate chunks and walnuts. Using a 1 1/2-inch scoop, scoop cookie dough, and place 1 1/2 inches apart onto prepared baking sheets.
Working in batches if necessary, place cookie sheets in freezer until cookies are thoroughly chilled, about 15 minutes. Bake chilled cookies in preheated oven until centers are just set, 10 to 11 minutes. Remove from oven, and let cool on baking sheets 5 minutes; transfer to wire racks to cool completely, about 20 minutes.