In a medium size saucepan, mix together sugar, cocoa and cornstarch over medium heat for about 2 minutes.
Add milk and whisk continuously until thick.
Remove the saucepan from the heat and add butter. Mix until butter is completely incorporated and allow to cool.
Pour pudding into a bowl and cover with plastic. Allow to chill in the refrigerator for at least one hour.
Fill a mason jar halfway with the chocolate pudding, then with a layer of graham crackers. Add another layer of pudding and top off with marshmallows, more graham cracker crumbs and a large piece ofHershey’s chocolate.