Stir rice flour and glutinous rice flour till evenly mixed.
240 g glutinous rice flour, 40 g rice flour
Dissolve sugar in hot water, stirring till water is warm, not hot. Add to flour. Mix well.
4 tbsp sugar (6 tbsp if not using filling), 140 ml hot water
Gather 85 g of wet dough (some flour would still be dry). Make into small discs. Cook in boiling water till floating. Mix with raw dough whilst still hot but cool enough to handle.
Knead till evenly mixed. If necessary, add a bit more warm water or glutinous rice flour so that dough is not too dry or too sticky.
Roll dough into a ball and set aside, covered, for 10 minutes. (This allows the flour to fully absorb the water added.)
Divide dough into 32 pieces. Roll each piece into a ball. Keeping balls not working on covered, fill with filling if using, around 1 tsp.
150 g filling (optional)
Make sure there's air in the dough, i.e. DON'T wrap dough tightly round filling. If there's no filling, it would be just an air pocket inside the dough.
Place sesame seeds in a bowl.
¼ cup white sesame seeds
Dredge filled rice balls in white sesame seeds. If rice balls are dry, dunk quickly in water or pat surface with a bit of water before dredging. Press gently so that sesame seeds stick well.
In a pot or wok, add enough oil to cover sesame balls, about 4 cm deep. Heat till oil is moderately hot. (Test oil temperature by putting an uncoated wooden chopstick in the oil. If there's no reaction, wait a few more seconds. If there's rapid sizzling and big bubbles, turn off heat to let oil cool down slightly. If there're small bubbles and gentle sizzling around the chopstick, the oil is just right. Reduce heat to very low. For gas stoves, the flame should be slightly flickering or just steady.) corn oil for deep-frying
Place glutinous rice balls in oil, not too many so that all can move around freely. Fry till sesame balls start floating, gently pressing any that doesn't expand evenly to get a round shape, with a spatula against the wok/pot or another spatula.
After rice balls start floating, increase heat to medium. (If the heat is too low at this stage, rice balls would be too soft and chewy inside.) Sizzling should increase from slow to moderate speed, but not too rapid. Stir gently to ensure even browning. Keep stirring and frying till rice balls are golden brown, about 10 minutes.
Remove sesame balls with a slotted spoon to drain on paper towels and cool down slightly. Serve hot or warm.