Split the bolillos in two and arrange on a rimmed baking sheet. Spread with butter. Place in the oven and broil for 1-2 minutes, or until the edges begin to crisp. Carefully remove from the oven.
Divide the refried beans among the toasted bolillos, about 2 tablespoons each. Smash them down with the back of a fork. Sprinkle half of the cheese over the beans.
Cut the avocado in half and remove the pit. Cut into quarters and peel off the skin. Slice each quarter into three sections. Place a slice of avocado on each bolillo bun.
Sprinkle remaining cheese all over the avocado. Place under the broiler and cook for 2-3 minutes, until the cheese is fully melted. Keep a close eye on the pan and do not walk away.
Remove from oven and cool molletes slightly. Serve hot with sour cream, salsa and sprouts or lettuce.