In a food processor or blender, puls the whole Oreos into fine crumbs. (You can also put the cookies into a freezer bag and crush them using a rolling pin)
Transfer the Oreo crumbs into a medium bowl, add melted butter and stir until the crumbs are evenly moistened.
Press the mixture in a tart pan with a removable bottom. ( I used a 7-inch tart pan, if using bigger you might need to increase the amount of the ingredients)
Place it in the fridge or the frizzer to firm up.
Chocolate Mousse
In a heatproof bowl add the dark chocolate and 60ml whipping cream. Place the bowl over a saucepan with simmering water, over low heat, until all the chocolate is melted
Manwhile, dissolve gelatin in cold water and let it swell for about 5 minutes. Place the gelatin over low heat just until the gelatin dissolves and pour it over the melted chocolate mixture. Let the chocolate mixture cool completely at room temperature.
Whip the remaining 140ml chilled whipping cream until stiff peaks form. Add melted chocolate mixture and mix until well combined.
Fill the mixture into the prepared tart pan, smoothing out evenly, and refrigerate for at least 4 hours or better overnight to firm up.
Chocolate Ganache
Put the dark chocolate in a bowl and heat the cream in a small pan until just steaming. Pour the cream over the chocolate and leave to melt for 5-10 mins. Allow it to cool at room temperature, then spread over the chocolate mousse.
Refrigiate for another 3min, or until the ganache has set.
To Serve
When the mousse is chilled and firm, you can remove the tart by gently pressing against the removable bottom of your pan. Decorate the tart with fresh berries or other toppings, serve chilled.