In a large mixing bowl, beat vanilla pudding mix with milk, until there are no lumps. Beat in peanut butter until smooth. Fold in the Cool Whip, set aside.
In a 9-inch square baking dish, layer 6 grahams. Top with half of the pudding mix. Top with 6 more crackers and the chopped Reese's PB cups. Top with the remaining of the pudding mix. Finish by topping with the remaining 6 graham crackers.
For the frosting, heat unsweetened chocolate with butter and corn syrup on medium heat in a small saucepan. Whisk until smooth. Remove from heat and whisk in the powdered sugar and milk. Frost the top of the graham cracker layer and place in refrigerator.
Refrigerate for 4 hours or overnight. Cut and enjoy!
Notes
Prepare it ahead of time. I like to make the cake the night before I plan to serve it. Chilling overnight gives eclair cake an even better flavor and texture!
Canned chocolate frosting tastes wonderful on eclair cake, too! Open the can, remove the lid and microwave for 20 seconds. Stir frosting, then pour it over the top layer of graham crackers.
Store eclair cake in refrigerator, covered, for up to 5 days. The graham crackers will soften more and more each day.
In order for the pudding mix to set properly in the cake, we recommend using cows milk (not oat or almond milk).