Wash, drain and pour into a pot. Add 1.5l of water to the red bean and bring to the boil. Add the pandan leaves. Once boiling, reduce to low heat and simmer for 2 hours, stirring occasionally.
After 2 hours, remove the pandan leaves. Add the rock sugar gradually, and stir to dissolve. Taste, and sweeten with more rock sugar if necessary.
Make the tang yuan dough. Sift the glutinous rice flour into a bowl. Gradually add water to form a stiff dough. Place dough on board and knead well for 3-4 minutes. Roll firmly into the size of a marble ball.
Drop the balls into the red bean syrup, and cook until they float to the surface. Serve each bowl of hot red bean soup with 3 tang yuans.