Step 1

In a medium bowl, combine the beef, sake, soy sauce, sesame oil, sugar, garlic and ½ teaspoon pepper. Mix well, then cover and refrigerate for 1 hour.

Step 2

Heat a 10-inch skillet over medium. Add the beef mixture and cook, stirring frequently and breaking it up into very small pieces, until no longer pink and all liquid released by the meat evaporates, 10 to 12 minutes. Do not allow the meat become crisp. Transfer to a large bowl and set aside to cool.

Step 3

While the beef is cooling, in a medium saucepan over medium-high, bring the rice and 2½ cups water to a boil. Cover, then reduce to low and cook until the water has been absorbed, about 12 minutes. Remove from the heat and let stand, covered, for 10 to 15 minutes.

Step 4

While the rice is cooking, use your fingers to further mash the beef into tiny pieces. When the rice is ready, fluff, then add to the beef. Add the seaweed (if using), 1 teaspoon salt and ½ teaspoon pepper, then stir until well mixed and just warm to the touch. Taste and season with salt and pepper.

Step 5

Lightly moisten your hands with water. Using a ⅓-cup measuring cup, scoop up the rice mixture, turn it out into your hands and form into a ball, compacting the mixture to hold together. Place on a large plate and repeat until with remaining rice mixture, lightly moistening your hands as needed to prevent sticking.

Step 6

Place the sesame seeds in a small bowl. Dip half of each ball into the seeds and set on a platter. Serve at room temperature with the sauce.