Place plantains in a large pot; cover with water. Bring to a boil, then reduce heat and simmer until tender, about 15 minutes. Drain and mash.

Heat refried beans in a small saucepan over low heat. Stir in sugar and salt. Remove from heat.

Form a palm-sized amount of mashed plantains into a ball; flatten and place about a teaspoon of bean mixture in the middle; then mold the sides of plantain around beans, making an egg-shaped ball.

Heat oil in a deep fryer or large skillet to 375 degrees F (190 degrees C), or until a small amount of batter sizzles and browns in oil.

Fry rellenitos in hot oil until browned. Drain on paper towels.