Powder or crush half of oreo biscuits, pour on a casserole then sprinkle expresso coffee and keep aside. Boil milk, condensed milk, cocoa and coffee powder with custard powder, whisk it when it boils remove from flame and cover it. Beat cream till peaks are formed, add mocha custard in it and fold in well. Pour mousse on oreo base and level it. Decorate with extra oreo biscuits, choc flakes and walnuts. Let it chill in fridge for three hours, then serve