Grate the yamato imo with a fine grater. If you have a suribachi (Japanese mortar with fine grooves on the inside), grate the yamato imo using a circular motion.
Add the water or sugar syrup in 2 to 3 batches, then mix with a pestle while incorporating air, and mix until it becomes sticky.
Add the sugar in 2 to 3 batches, and mix well with the pestle and mortar.
Put the jyoshinko in a bowl, then add Step 4. Fold the paste in half, then press it into the flour, and repeat, mixing it in a little at a time. (Refer to Helpful Hints.)
If made correctly, the dough will be elastic like marshmallows, and will make little popping sounds when stretched.
Divide the Step 5 dough into 10 uniform portions. Roll the anko paste into 10 balls that are 20 g each. Stretch the dough into a circle, then wrap it around the anko paste, seal, then adjust the shape.
Bring water to a boil in a steamer, put in Step 7, mist with water, then steam for 10 minutes on high heat. (cover the steamer with a cloth to catch the condensation).
After steaming, transfer to a wire rack and cool with a fan to form a gloss on the dough.
They're done! Here they are with food coloring brushed on top.
Story Behind this Recipe
I made these from ingredients I had on hand.
If made correctly, the dough will be elastic like marshmallows and will make little popping sounds when stretched apart. Test the dough often while mixing; if it's done before all of the flour is incorporated, stop and do not try to mix it all in (this is key.)