In a large bowl, combine the instant vanilla pudding and cold milk. Whip until the pudding thickens and the mixture is smooth. Refrigerate for at least 30 minutes.

If using Cool Whip, fold it directly into the chilled pudding mixture. If using heavy whipping cream, whip it until stiff peaks form, then gently fold it into the pudding.

To make the chocolate ganache, heat 2/3 cup heavy whipping cream in the microwave until hot but not boiling. Pour it over the chocolate chips and let it sit for about 5 minutes. Stir until completely smooth. If the chocolate hasn’t fully melted, microwave it in short intervals, stirring between each one.

In a deep 9×13-inch pan, arrange a layer of graham crackers on the bottom.

Spread half of the pudding mixture evenly over the graham crackers. Add another layer of graham crackers, followed by the remaining pudding mixture. Finish with one more layer of graham crackers.

Pour the chocolate ganache over the top and spread it evenly.

Refrigerate for at least 4 hours, preferably overnight, so the graham crackers have time to soften and the cake can set.