Bring the water up to a boil and add the rice. Cook the rice until very soft and mushy, about 8 to 10 minutes. Remove the rice and let the rice cool completely. When the rice has cooled, mash the rice well. Dissolve the yeast in the warm water. Add the yeast to the mashed rice and mix well. Cover the bowl with plastic wrap and let the mixture sit overnight to rise. The next morning, whisk the eggs together. Add the eggs to the rice mixture. Add the sugar and flour to the rice mixture and mix well to form a thick batter. Let the mixture sit for 15 minutes. Preheat the oil in a stock pot. Drop a heaping spoonful of the batter into the hot oil. Fry in batches. Fry the calas until golden brown, about 3 to 4 minutes. Remove the calas from the oil and drain on a paper-lined plate. Repeat the process until all the calas are fried. Sprinkle the calas with powdered sugar and serve immediately.