Combine crumbs with diet margarine by cutting in softened margarine until mixture resembles coarse crumbs
Press firmly in bottom and sides of 8 or 9 inch pie pan
Bake in preheated 350 degree oven for 8 to 10 minutes
Cool
In small saucepan, sprinkle gelatin over 1/2 cup skim milk
Let stand one minute
Heat, stirring constantly until gelatin dissolves
In blender or food processor, blend ricotta until smooth and add gelatin mixture, remaining 1 cup milk and vanilla
Continue blending until completely smooth
Remove half the mixture, set aside
To mixture still in blender, add 6 packs sugar substitute and cocoa
Blend thoroughly
Pour blender mixture into crust, chill for 30 minutes or until partially set
At the same time, chill remaining mixture for 30 minutes
Prepare whipped topping mix according to package directions gradually adding remaining 6 packets sugar substitute
Whisk into reserved, chilled mixture until blended smoothly
Spoon over chocolate layer; chill until set
Garnish with dusting of cocoa