Crush 350 g of the Oreo cookies into small pieces.
Melt the butter, pour it over the crushed cookies and mix well.
Line the bottom of a 24 cm round springform pan with baking paper.
Press the Oreo mixture evenly into the bottom of the pan.
Break the remaining Oreo cookies into smaller pieces and set them aside for later.
Beat the Philadelphia cream cheese and powdered sugar together until smooth and creamy.
Whip the heavy cream with the sugar until soft and fluffy. Add it to the cream cheese mixture and mix gently until well combined.
Spread half of the cream cheese mixture evenly over the Oreo base. Place the cake and the remaining cream cheese mixture in the fridge for about 30 minutes while you prepare the chocolate mousse.
Melt the milk chocolate gently over a water bath, then set it aside to cool slightly.
Whisk the egg and sugar together until light and fluffy.
Whip the heavy cream until soft and fluffy.
Add the egg mixture to the whipped cream and mix gently until combined. Add the cooled melted chocolate a little at a time, mixing gently until the mousse is smooth and evenly combined.
Spread the chocolate mousse evenly over the cream cheese layer.
Sprinkle the reserved Oreo pieces evenly over the chocolate mousse.
Place the cake in the fridge for about 1 hour, or until the chocolate mousse has set.
Spread the remaining cream cheese mixture evenly over the chocolate mousse and Oreo layer.
Place the cake back in the fridge and let it chill for at least a couple of hours before serving. For the best flavour and texture, I recommend leaving it in the fridge overnight-