Place the beans and water in a saucepan, cover, bring to boil over medium high heat. Reduce heat to medium-low and cook for 1 1/2 hours, or until beans are very soft. Strain the water from the beans, and blend them into a puree in a blender or food processor. Press the puree through a sieve, discarding the skins, which will be left in the sieve. Place the puree in several layers of cheesecloth, and gently squeeze to remove excess moisture. Place the thickened puree back into the saucepan, together with the sugar and vegetable shortening, and heat over low heat, stirring until it becomes a thick paste. Stir and scrape vigorously so that it does not stick to the bottom. Remove from heat and let cool before filling buns.
Combine the rice flour and water a little at a time until you have a crumbly dough. If you take a small amount of the dough in your hand and squeeze it and it stays together, then you have enough water in there.
Take a generous tablespoon of dough and roll it into a ball in your hands. Make a hole in the middle of the ball of dough with your thumb - it needs to be big enough to hold about a teaspoon of the bean paste.
Put the bean paste into the dough and gently press to seal.
If the dough seems a little dry, moisten it just a little, then roll the dough in the sesame seeds, making sure you've got plenty of seeds on there.
Fry 3 or 4 balls at a time in oil at 350 degrees until the dough is cooked through and the sesame seeds are golden brown - this should take 3 or 4 minutes.