Onigiri (Japanese Rice Balls)

Ingredients for 10 servings

• 2 cups (360g) Japanese short-grain rice
• 1/2 teaspoon Diamond Crystal kosher salt; if using table salt, use half as much by volume1 1/2 cups (355ml) warm water, about 90ºF (32ºC)Suggested fillings: tuna-mayo, finely shredded salted salmon (shiozake), umeboshi, and/or another filling of your choice (see notes)
• 2 sheets nori, cut into ten 1- by 4-inch rectangles, plus additional sheets for wrapping (optional)
• 1/2 cup (59g) furikake (Japanese rice seasoning), optional