Protein is the most important nutrient for muscle growth, but without carbs and healthy fats, you won't get far. You need all those three macronutrients to build muscle mass. You are going to need a high protein intake (1 - 1,2 g of protein for each pound of your body weight), and twice as high carbohydrates intake.
Fats from foods like avocado, olive oil, and nuts are also recommended as they are very healthy and high in calories. You will consume 8 - 9 calories for each one gram of fat, so by eating 100 grams of peanut butter, you will consume 50 grams of fat which is equal to 400 - 450 calories. That amount of peanut butter also provides 25 g of protein and 20 g of carbohydrates.
Don't be afraid of healthy fats, they can be very helpful to hard gainers to gain lean muscle mass faster. Saturated fats that are found in dairy and meat products may increase testosterone levels and may also promote strength and muscle gain.