Arrange the racks in the upper and lower third of the oven and preheat to 350 degrees F. Line 2 baking sheets with parchment paper.
Stir together the brownie mix, butter and egg in a large bowl until smooth and combined, then fold in the chocolate chunks.
Scoop 24 balls of dough (about 2 tablespoons each) onto the prepared baking sheets. Use the palm of your hand to gently press each ball into a thick disk. Bake until the cookies spread and the edges are just set, rotating the trays halfway through, 10 to 12 minutes. Cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely.