Combine the dark brown sugar and 50 ml water in a small 1-quart / 1L saucepan. Simmer over low heat for 3 to 4 minutes, stirring, until syrupy and slightly darker. Set aside to cool.

In a 2-quart / 1.9L saucepan, whisk the kuzu starch into the 400 ml cold water until fully dissolved with no lumps.

Set the pan over medium-low heat. Stir constantly with a rubber spatula, scraping the bottom and sides, for 8 to 10 minutes.

Keep stirring until the mixture shifts from cloudy white to translucent gray and turns thick enough to mound on the spatula without dripping off. Don't stop stirring during this stretch, even 30 seconds unattended can let it clump and scorch on the bottom.

Pour the hot mixture into an 18 cm / 7-inch square dish. Smooth the top with a wet spatula.

Set the dish into a large bowl of ice water for 15 minutes, until the surface feels firm to the touch.

Cover and refrigerate for at least 1 hour, until fully set through the center.

Turn the set kuzumochi out onto a damp cutting board. Cut into 2.5 cm / 1-inch cubes with a wet knife, wiping the blade between cuts.

Divide the cubes into bowls. Dust generously with kinako and drizzle with the kuromitsu right before serving.