In a large bowl, whisk together the milk and instant pudding mix until smooth and slightly thickened. Fold in the Cool Whip until fully combined.
Arrange a single layer of graham crackers in the bottom of a 9×13-inch baking dish. Spread half of the pudding mixture evenly over the crackers, then add another layer of graham crackers.
Spread the remaining pudding mixture over the second layer, then finish with one final layer of graham crackers.
Remove the lid and foil seal from the chocolate frosting and microwave it for about 30 seconds, stirring until smooth and pourable. (You can also transfer it to a microwave-safe bowl before heating.) Spread the frosting evenly over the top layer of graham crackers.
Cover and refrigerate for at least 6 hours, or overnight, to allow the graham crackers to soften. For cleaner, more defined layers, freeze the cake overnight, then thaw it in the refrigerator for at least 3 hours before serving.
Notes
Refrigerate overnight for the best texture and flavor.
Warm the frosting slightly so it spreads easily over the graham crackers.
Freeze overnight for cleaner slices, then thaw in the refrigerator before serving.
Use a 9×13-inch pan for even layers.
Be patient; the longer it chills, the better it gets.