Place the top portion of a double-boiler over hot water, add butter to the bowl, and then set the heat to low. When the butter has melted, add the chocolate chips, reserving 1 tablespoon of the chocolate chips and set aside. Melt chips, 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 a clean 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. Put the chocolate wafers into a plastic bag and using a kitchen mallet crush to make crumbs.
To Prepare Dessert:
Spoon 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. Remove lid from fudge topping and warm up the fudge in the jar in your microwave. Top with a rosette of whipped cream on top and sprinkle it with leftover Oreo crumbs.