Well, fats are always gonna be denser in calories than other types of food. Make sure to get a well-rounded intake of all macronutrients. With that said, some good high-calorie foods are nuts, seeds, cheeses, whole milk, butter (if you really want to) and olive oil. Like I said though, the calories in these foods are dominantly fats. People suggest the gallon of milk a day since whole milk has a good distribution of fats, carbs, and proteins. Also, it's easy to stomach because it's a liquid and doesn't leave you feeling as full as a solid food with such a high level of calories.