Place 18 Oreos in a food processor and pulse until finely ground. If you don't have a food processor, place cookies in a zip-top bag and crush with a rolling pin until fine.

Transfer crumbs to a bowl and stir in 4 tbsp melted butter until the mixture resembles wet sand and holds together when pressed.

Divide the crumb mixture evenly among 6 serving glasses or dessert cups (roughly 6–8 oz / 180–240 ml cups). Press the crumbs firmly into the bottom (about 1–1.5 tbsp per cup, depending on cup size) to form a compact base. Chill in the refrigerator while you make the mousse (10–15 minutes).

Chocolate mousse

Melt chocolate

Place 150 g finely chopped dark chocolate in a heatproof bowl. Heat 120 ml heavy cream in a small saucepan just until it begins to simmer (do not boil). Pour the hot cream over the chopped chocolate. Let stand for 1 minute, then stir gently until smooth and fully combined. Set aside to cool slightly (5–10 minutes) until warm, not hot.

Prepare cream cheese mixture

In a medium bowl, beat 60 g room-temperature cream cheese with 30 g granulated sugar and 1 tsp vanilla extract until smooth and no lumps remain. Scrape down the bowl.

Combine chocolate and cream cheese

Fold the slightly cooled chocolate ganache into the cream cheese mixture until evenly combined and smooth. Ensure the chocolate is not hot, or it will melt the whipped cream.

Whip cream

Using a chilled bowl and whisk (or stand mixer), whip 300 ml cold heavy whipping cream to soft-medium peaks. Start at medium speed; when the cream thickens and holds a soft peak, increase speed briefly to reach medium peaks — the peaks should hold shape but still be slightly soft at the tip.

Reserve about 6 tbsp of the whipped cream for topping (optional). Gently fold the remaining whipped cream into the chocolate–cream cheese mixture in three additions. Use a spatula and fold carefully to preserve air: add one third, fold until mostly incorporated, then repeat until the mousse is uniform and light.

Assembly & garnish

Remove the prepared bases from the refrigerator. Spoon or pipe the chocolate mousse over each Oreo base, filling the glasses almost to the top. Smooth the tops with a spatula or the back of a spoon.

Top each mousse with a spoonful of the reserved whipped cream (or pipe decorative rosettes), then sprinkle crushed Oreos (from 4 extra cookies) over the tops. Garnish each serving with a mini Oreo or half-cookie, if using, and dust lightly with 1 tsp cocoa powder if desired.

Chill the assembled mousses in the refrigerator for at least 2 hours (preferably 3–4 hours) to set and let flavors meld. Serve cold. Leftovers can be refrigerated, covered, for up to 2 days.