In a medium bowl, mix together the mochiko, 1 cup of sugar, and water until well blended. Mix in the 2 drops of vinegar to soften. Pour into a microwave-safe dish, and loosely cover with plastic.
Microwave for 2-3 minutes, mixing every 30 seconds. Remove, and let cool until cool enough to handle.
In a small bowl, combine the potato starch, 1/4 cup sugar, and salt. Turn the mochi out onto the plastic, and cut into pieces using a plastic or wooden knife. Metal knives tend to stick too much. Roll the pieces in the potato starch mixture.
Coat the inner surface of the custard cups with unsalted butter.
Make caramel sauce:
Put the sugar and water in the pot. Heat the pot at medium heat (low heat will not work).
Let it sit until the surface becomes lightly-colored. Swirl the pot and even out the sugar liquid.
Caramelize the sugar and remove the pot from the heat.
Quickly add the hot water in 2 to 3 steps with a long-handled ladle. Tilt the pot away from you to avoid hot caramel sauce splashing and burning your hand. Swirl the pot and even out the hot caramel.
Pour the caramel evenly into the custard cups.
Crack the two eggs into a bowl. Lightly beat the egg with a balloon whisk. Don’t let the tip of the whisk leave the bottom of the bowl to avoid creating too much foam.
Add the milk and sugar in a pot. Turn on the burner. Stir with a spatula until all the sugar is dissolved.
Add the milk to the beaten egg while whisking the mixture.
Sieve the egg mixture with a fine mesh strainer. Scrape off the bottom of the strainer with a spatula. Remove any foam on the surface with a spoon. Pour the egg mixture into an easy-to-pour container.
Gently fill the custard cups with the egg mixture.
Cover each cup with aluminum foil. The foil will prevent the surface from getting dry. It will also help steam the pudding evenly.
Heat water in a pan and gently place the cups in it. The kitchen towel will soften the heat at the bottom. There should be enough hot water to cover the bottom half of the cups. Put the lid on.
Keep the water temperature just below the boiling point and steam the pudding for 18 to 20 minutes.
Do not bring the water to boil otherwise the smooth texture of the pudding will be lost.
Put kitchen gloves on and remove the custard cups. When they become cool, store them in the fridge.
Let’s serve the custard pudding. Scrape the side of the cup with the tip of a knife. Tap the cup on a kitchen towel and make a space around the pudding. Serve the pudding on a plate.
