While no specific food is shown to fill out your rear, gaining weight will make your entire body grow larger. You'll accomplish this by eating more calories than you burn for energy, causing your body to store the excess as fat. Eating 500 extra calories per day will make you gain 1 pound each week. (ref 1)
Eating for Weight Gain
The best foods to choose for weight gain are those that pack a large number of calories into a relatively small serving, allowing you to eat more calories without becoming stuffed. Healthy options include peanut butter, nuts, granola, reduced-fat cheese, avocado, dried fruit and dense whole-grain breads. Also drink caloric beverages, such as smoothies and low-fat milk, instead of water to add more calories to your diet. (ref 1, graf 4)
Food may not target your buttocks, but strength-training exercises do -- and performing them may help create a rounder, fuller shape. Try squats and lunges, or walk up steps with a weight in each hand. (ref 2) For balance, you should perform strength-training exercises for your other muscle groups, as well.