How to Make It
Place 1/2 teaspoon salt in a blender. Tear or cut nori sheets into 1- to 2-inch pieces; add to blender. Process on medium-high speed until mixture resembles graphite-colored glitter, about 1 minute. Transfer to an airtight container.
If using hot, freshly cooked rice, spread in an even layer on a baking sheet lined with paper towels; let cool until rice is room temperature and grains are dry, about 30 minutes. If using chilled day-old rice, spread in an even layer on a baking sheet lined with paper towels; let come to room temperature. Gently rub rice back and forth between your hands to separate the grains. Place rice and remaining ingredients near stove so you can cook quickly.
Heat 2 tablespoons oil in a 12-inch carbon steel or nonstick skillet over medium. When oil is nearly shimmering, add garlic, and cook, stirring constantly, until garlic is toasted and fragrant, 10 to 15 seconds. Add shrimp, and stir to combine. Season with remaining 1/4 teaspoon salt. Cook, stirring constantly, until shrimp are nearly cooked through, 45 seconds to 1 minute.
Add 1 1/2 cups rice to skillet, and stir to combine. Increase heat to high. Cook, undisturbed, 20 seconds; stir. Repeat cooking-and-stirring process until a dry skin forms around some of the rice grains, 2 to 3 minutes. Transfer mixture to a medium bowl. Repeat procedure with remaining 1 tablespoon oil and remaining 11/2 cups rice. Return rice-shrimp mixture to skillet, and stir to combine. Drizzle with fish sauce, and cook, stirring constantly, 20 seconds. (This will ensure rice grains are well-seasoned.)
Remove from heat. Stir in sliced scallion, and let stand until scallion is just softened, 10 to 15 seconds. Stir in 1 tablespoon nori dust.