For crust: Finely crush Oreos in a food processor. Add the melted butter and mix well. Press into the bottom of 12 paper lined muffin cups.
For filling: Beat cream cheese in a mixing bowl till smooth. Gradually beat in the sweetened condensed milk, then the egg and vanilla.
Divide filling between the 12 crusts. Bake at 350° for 20-25 minutes or till puffed up and starting to crack. (They will sink as they cool.)
Remove to cooling racks. Cool completely, then chill for at least 30 minutes.
For mousse: Melt chocolate in a glass bowl in the microwave at half power, stirring every 30 seconds. Set aside to cool slightly.
In a glass mixing bowl, beat cream and powdered sugar until soft peaks form. Quickly beat in the melted chocolate. Fold in the crushed Oreos. Add enough milk to make a spreadable mousse.
Using a LARGE tip, pipe the mousse on top of the cheesecakes. (You need a large tip or the Oreos clog it up.)