In a medium bowl, whisk together the vanilla pudding and 3 1/2 cups milk for about 2 minutes. Fold in Cool Whip.
Arrange a single layer of graham cracker squares in the bottom of a 9x13-inch pan. You may need to cut and shape the crackers to fit snuggly.
Evenly spread half of the pudding mixture over the graham crackers.
Top with another layer of graham crackers to cover entire surface.
Pour the remaining pudding mixture on top of the crackers and smooth evenly.
Place the final layer of graham crackers on top with bumpy side down. (This will help the top look smooth when you put the ganache on top.)
CHOCOLATE GANACHE: In a medium glass bowl, microwave butter and 6 tablespoons of milk until melted, not boiling. Whisk in cocoa and confectioners' sugar, beat until smooth. Pour ganache through a sieve to remove any lumps. Carefully cover the cake with ganache, spreading evenly.
Chill for at least 6-8 hours or overnight.
Cook's Note: The ganache will be a bit runny. No worries, it will firm up in the refrigerator.