Rinse and drain the rice. Place the short-grain rice in a bowl, cover with cold water, gently swish and drain. Repeat 3–4 times until the water is less cloudy. Drain in a fine-mesh strainer for about 5 minutes. Use Japanese short-grain rice so the grains hold together when shaped; don't substitute long-grain rice.
Cook the rice. Combine the rinsed rice and 1 2/3 cups (400 ml) water in a saucepan with a tight-fitting lid. Bring to a simmer, then cover, reduce the heat to low, and cook for about 15–18 minutes, or follow package directions, until the water is absorbed. Turn off the heat and let stand, still covered, for 10 minutes. Fluff gently, without crushing the grains. Keep the lid on during cooking and resting; rice brands and stovetops may need slightly different water or timing.
Cook the miso-leek filling. While the rice cooks, rinse the sliced leek thoroughly and drain. Melt the butter in a skillet over medium-low heat and cook the leek for about 6–8 minutes until soft, stirring often. In a small bowl mix the white miso with 1 tablespoon water and the soy sauce; add to the skillet and stir for 1–2 minutes until the mixture coats the leeks and no loose liquid remains. Set aside to cool slightly. Do not boil or brown the miso aggressively; the filling should be moist but not watery.
Prepare the shaping station. Cut one nori sheet into four short bands and small optional face details. Put a bowl of clean water and the salt near the work surface. Divide the warm rice into four equal portions; keep it covered while you work. Shape while the rice is warm enough to cling but cool enough to handle safely; a clean onigiri mold or plastic wrap can help.
Fill and shape four onigiri. Lightly wet your clean hands and rub on a little salt. For each portion, place about half the rice in one palm, make a shallow well, and add about one-quarter of the miso-leek mixture (roughly 1–2 tablespoons). Cover with the remaining rice from that portion and gently press into a rounded triangle or oval, sealing the filling inside. To make cats, gently pinch two ear points at the top without crushing the grains. Don't overfill or squeeze too tightly; keep the filling enclosed and use damp hands to prevent sticking.
Decorate and serve. Press a nori band across the front of each onigiri and add small nori facial details if desired. Serve one large onigiri per person while fresh. If not eating promptly, cool in shallow portions and refrigerate without leaving cooked rice at room temperature for an extended period. Add the nori close to serving so it stays pleasantly crisp.
