Make a double-boiler by setting a metal bowl on top of a medium saucepan filled with a couple inches of water. Add butter to the bowl, and then set the heat to low. When the butter has melted, add the chocolate chips, except for 1 tablespoon that you set aside. When chips are melted, add chocolate liqueur and vanilla. Remove bowl from the heat and whisk in egg yolks.
In a separate bowl beat egg whites until they form soft peaks. Stir egg whites into chocolate mixture.
In another medium bowl, combine sugar with cream, and then whip cream until stiff. Fold cream into chocolate.
Chop remaining 1 tablespoon of chocolate chips into small bits the size of rice. Stir this chocolate into the mousse, then cover and chill the mousse for 3 to 4 hours.
Open the Oreos and scrape out the filling. Pop the chocolate wafers into a plastic bag and pound on it with a kitchen mallet until you've got a bag full of crumbs.
Make each serving by spooning 2 tablespoons of Oreo cookie crumbs into the bottom of a 6-ounce rocks glass. Spoon 1/3 cup of chilled mousse into the glass, and top it with 2 tablespoons of chocolate fudge topping.
Warm up the fudge a bit in the jar in your microwave so that it's easy to spoon into the glass. Spray a pile of whipped cream on top and sprinkle it with some of the leftover Oreo crumbs.
Servings: 4 servings.