Line a 9×13-inch pan with parchment paper, leaving an overhang for easy removal. Preheat the oven according to the fudge brownie mix package directions.

Prepare the 18.4-ounce fudge brownie mix according to the package directions. Pour the batter into the prepared 9×13-inch pan.

Bake the brownies according to the package directions but remove them from the oven 3 minutes before the package’s recommended time (slightly underbaked). The brownies should be set but still soft. Let the brownies cool in the pan about 20 minutes.

While the brownies cool, set aside 3 Oreos. Coarsely chop the remaining 24 Oreos (for example, cut each into 4 pieces).

Gently press the chopped 24 Oreos in an even layer over the cooled brownie surface.

Warm the entire 11.75-ounce container of hot fudge topping in the microwave about 30 seconds (microwave times vary). Stir and drizzle dollops of the warm hot fudge over the Oreo layer, then use the back of a spoon to spread the fudge into an even layer.

Place the pan in the freezer for 10 minutes to set the hot fudge.

Remove the 48-ounce container of cookies and cream ice cream from the freezer and let it sit at room temperature about 5 minutes to soften slightly.

Remove the pan from the freezer. Scoop large portions of softened ice cream onto the frozen hot fudge layer and use a spatula to gently press and smooth the ice cream into an even layer.

Freeze the pan for at least 2 hours (preferably overnight) until the ice cream is firm.

Spread the completely thawed 8-ounce container of frozen whipped topping evenly over the firm ice cream. If desired, garnish with the 3 reserved Oreos (whole or chopped). Cover the pan and return it to the freezer for 30 minutes to set the topping.

To cut bars: let the pan sit at room temperature about 5 minutes to soften slightly. Use the parchment overhang to lift the slab from the pan. Wipe a sharp knife dry between slices — warm the knife under hot water, dry it, then cut. Return any leftovers covered to the freezer immediately.