Line the base of a 20cm diameter (7 – 8 Inch) deep, round springform baking tin with foil. Brush oil over sides and base.
For the base, break cookies into smaller pieces and blend in a food processor to a fine, sand like consistency. Transfer to a bowl and pour in melted butter. Stir until well combined. Press the mixture into the base of the baking tin, distributing evenly. Refrigerate for 20 minutes until set.
Place the set base in a preheated oven of 160C (320F) and bake for 15 minutes until golden. Remove and set aside to cool.
For the mousse, whip the cream until soft peaks form. Melt the chocolate in a heat proof bowl over a saucepan of gently simmering water. Do not let the water touch the bowl. The chocolate needs to slowly melt, not cook.
Once the melted chocolate has cooled slightly, pour into the whipped cream and fold in gently until combined and the chocolate is completely blended into the cream. Pour over the cooled base, smoothing the surface. Cover with cling wrap and place into the refrigerator for at least 6 hours to set.
At this stage the cake can be served with a dusting of cocoa powder and topped with berries. Otherwise, see below for the ganache.
For the ganache, combine the chocolate and cream in a heatproof bowl over gently simmering water, stir with a metal spoon until just melted and combined. Place in the refrigerator to thicken (around 15 minutes) then pour ganache over chilled mousse cake and return to the refrigerator to set further or until ready to serve.
When removing sides of baking tin, use a hot knife and slide around the edge of cake and tin to separate more easily. Sprinkle with shaved white chocolate.