Mix oreo crumbs, 1/4 cup sugar and 1/4 cup melted butter. Pack into a 9x13 dish.
In largest bowl of mixer, beat cream cheese, sugar and vanilla thoroughly.
When lumps are gone, beat in cocoa.
Fold in whipped topping, just until mixed, on low speed.
Fill crust, cover with plastic wrap and chill for 3 hours.
Sprinkle with extra cookie crumbs if desired.