Preheat oven to 350 degrees F (175 degrees C). Grease 12 muffin cups or line with paper liners.

Mix brownie mix, oil, eggs, and water together in a bowl until smooth. Spoon about 1 tablespoon batter into each muffin cup. Place 1 cookie into each muffin cup and top with more batter.

Bake in the preheated oven until a toothpick inserted in the side of the brownie comes out clean, 20 to 23 minutes.