fats are good for you.. and isn't chocholate mostly carbs? I dunno..
To gain weight you need to get in a calorie surplus, that means eating more calories than you burn during the day. You can easily do this with regular food. If you want a suppliment I would go with protein only since (like you said) you can much more easily get carbs from the diet (not that it's hard getting protein from regular food, but it's time consuming and expencive to eat a ton of meat just for the protein)