Heat the oven to 350°F (175°C) and grease 24 miniature muffin cups. Divide the refrigerated cookie dough into 24 portions, then press each portion over the bottom and slightly up the sides of a muffin cup.

Bake the cookie shells for 10 to 12 minutes, until the edges are lightly browned. Keep a close eye on them because the centers should remain slightly soft rather than dry or brittle.

Let the cookie cups cool in the pan for about 5 minutes. Carefully transfer them to a wire rack and allow them to cool completely before adding the mousse.

Whisk the instant chocolate pudding mix with the cold milk for about 2 minutes, or until the mixture thickens. Scrape the sides and bottom of the bowl so the pudding powder hydrates evenly.

Fold the thawed whipped topping into the thickened pudding until the mousse is evenly blended and no obvious white streaks remain. Use a gentle folding motion to keep the mixture light.

Spoon or pipe the mousse into the cooled cookie cups. Refrigerate the filled cups until serving so the mousse is cool and pleasantly firm.