How is your family heredity? If your parents are also less weight then you have that heredity. How is your height? What is your activity style now?
All these also need to be considered for a healthy and systematic weight gain.
Theoretically speaking additional 7700 calories can increase your weigh by one kg. Now you are 43 kg and you need to gain 12 more kg to reach at 55 Kg. That means you need to take extra (7700 x 12) calories or 92400 calories.
But it is not practical or advisable to gain weight in extra fast time. Keep a reasonable goal of gaining that weight. Say, if you want it by six months, then each month you have to gain 2 kg or need additional 15400 calories. Divide it by 30 and yo may need about 515 calories every day without losing by extra workout. This is not very difficult.
You need only about 175 calories additional for each food take like breakfast, lunch and dinner.
(You can have your goal say 4 months or 8 months as suitable to you. But do not overdo.Let it be systematic and gradual.Watch for any wrong reaction,stomach problem, etc and take measures)
Depending on what you take usually you may take some items extra, some fat and oil or fried items, ice cream, or any other item . They need not be same every day, You can have variety items as per your taste and ability to digest. Take them happily. You may also go for some popular and reputed protein supplements .
Taking only extra food and no workout may create problems of fat accumulation. Hence you may have to dome proper physical activity also. Accordingly the food intake quantum or quality may have to be adjusted.
You may consult your doctor in this regard as he/she may know about your health matters better and custom suggest for you.