Sift rice flour, plain flour, salt and baking powder into a mixing bowl. Make a well in the centre and pour in the water. Mix well to form a smooth batter. Set batter aside to stand for 10 to 15 minutes.
Add in the slaked lime just before use. (If slaked lime is not available, use a teaspoon of bicarbonate of soda.) Sandwich a slice of nien koh in between a slice of sweet potato and a slice of yam.
Hold together tightly and dip into the batter. Drop into heated oil and deep-fry till golden brown. Drain well before serving.