Combine Warabi mochiko starch, sugar, and water in a pot. Cook at high heat, stirring constantly. When the mixture starts getting lumpy, turn down the heat to medium, and continue cooking and stirring until it becomes thick and translucent.
Moisten a small baking sheet with water, and transfer the hot warabi mochi mixture to a baking sheet. Spread a little, let cool to room temperature, and refrigerate for 1 hour.
For Kuromitsu sauce, combine all the ingredients in a small pot. Let boil for 3 minutes or until very slightly thickened. It will thicken further when completely cooled.
Mix Kinako soy powder and sugar together.
Right before serving, cut the warabi mochi into squares, coat with the kinako mixture, and top with Kuromitsu.
