Peel the sweet potatoes and cut into 1 cm chunks. Let soak in water for about 30 minutes and rinse to remove the scum. If you let it soak longer, the color will become a nice yellow.
Drain and transfer to a bowl. Coat with the sugar and let sit for about 1 hour. While doing this, some moisture will come out, but don't discard it.
Mix together the ★ ingredients. Add to the bowl around the sweet potatoes and let the ★ ingredients dissolve into the water that came out of the sweet potatoes.
Add the salt and mix well. Mix the sweet potatoes as well.
Line the steamer with parchment paper and place ball-sized lumps of the ingredients on top. Steam over high heat for 20 minutes.
I don't have a steamer, so I steam them in an earthenware pot. After about 10 minutes, there's no more water, so at about the 10 minute mark, add some hot water.
When the sweet potatoes can easily be pierced through with a skewer and the outside of the potatoes are shiny, it's done!
Story Behind this Recipe
My mother taught me how to make this. My mother's recipe uses baking powder as well, but I didn't use it this time.
If you don't have shiratamako, you can just use cake flour. I use light brown sugar, so the pictured example is a little brown. I received a tip on improving the brightness of the yellow: after removing the scum in Step 1, try adding in a little baking soda!