In a medium bowl, mix together Cool Whip, pudding mix, and 3 cups milk.
In a 9x13 inch pan, place one layer of graham crackers
Spread ½ of the pudding mixture over the top
Place another layer of graham crackers
Top with remaining pudding mixture
Place a final, third layer of graham crackers
In a microwave bowl or small saucepan, melt butter and chocolate together
Stir in 3 cups of powdered sugar, 1 tsp vanilla, and ½ cup milk
Continuing stirring until combined and no lumps remain
Pour chocolate mixture onto top layer of graham crackers, and spread in an even layer using the back of your spoon
Chill in refrigerator for 10-12 hours