For best results, begin preparing this dish the night before.
Place the water, sugar and yeast in a medium non-reactive bowl and allow to stand until foam sets, about 10 minutes. Add the rice to the water and yeast and stir to combine. Cover the bowl with plastic wrap and allow to proof at room temperature for at least 4 hours and up to overnight (8 hours).
The next morning/day stir the rice and mash it against the side of the bowl with the back of a spoon. You want to create a contrast in texture, with some grains remaining whole and some crushed. Add the eggs, salt, vanilla, nutmeg and flour to the bowl and stir with a spoon to combine. Cover the bowl again with plastic wrap and allow to proof in a warm, draft free place for 1 hour.
Preheat at least 4 inches of oil in a Dutch oven or a deep fryer to 360 degrees F. Use a large spoon or scoop to drop about half of the dough by 2-tablespoonfuls into the hot oil. Cook until golden brown on one side, about 2 minutes, turn over and continue to fry until cooked through, about 1 1/2 minutes longer. Using a slotted spoon, remove the calas from the hot oil and transfer to paper towel-lined plates to drain briefly. Sprinkle the calas liberally with the powdered sugar before serving hot. Repeat with the remaining batter.