Heat the oven to 350°F and line baking sheets with parchment. Beat softened butter with both sugars for about 2 minutes until pale, then incorporate the 2 cookie-dough eggs and vanilla.

Blend baking soda, salt, and the 2 1/2 cups flour into the butter mixture just until no dry flour remains. Fold in mini chocolate chips.

In a second bowl, combine the entire 18-ounce brownie mix with 1 1/4 cups flour. Stir in the 3 brownie-dough eggs and 1/3 cup oil until a thick dough forms; do not add water or follow the brownie-box batter instructions.

Divide each dough into about 36 equal portions. Press one light and one dark portion together for each cookie, keeping both colors visible, and leave at least 2 inches between cookies on the sheets.

Bake one sheet at a time at 350°F, checking at 9–10 minutes. Remove when the pale edges are lightly golden and both centers are set without wet dough; extend the bake if needed. Allow about 30 minutes for 3 baking batches, longer if your sheets hold fewer cookies.

Rest each batch on its sheet for 3–4 minutes before moving to a rack. Allow the last batch about 20 minutes to cool before packing; serve after the cookies are firm enough to lift.