Step 1) Into a blender and 15 oreo biscuit and powder. Then add melted butter and grind it again to make a paste like consistency. Spread it on to a pan and press it flat with a spatula.
Step 2) Dark chocolate mousse:
Mix gelatin in water and set it aside for 5 minutes, then heat it for few minutes until it becomes watery.
Now in a bowl add chilled heavy whipping cream and beat until stiff peaks. Add the whipped cream in bathes to the cooled chocolate mixture and gently fold it to mix evenly. Pour the dark chocolate mousse over oreo biscuit layer and cool it in the refrigerator until next layer is ready.
Similarly do with milk chocolate and white chocolate.