Combine the egg yolk and water in a small bowl and whisk well. Cover the bowl with a small plate and microwave on high for 10 seconds, or until the mixture starts to expand. Continue to cook 5 seconds longer. Whisk with a clean whisk. Cover and return to the oven 5 seconds. Whisk with a clean whisk, cover and let sit 1 minute.
Makes 1 1/2 cups.