How to Make It
Line a rimmed quarter sheet pan with plastic wrap, leaving 2 inches of overhang on all sides. Place rice in a large bowl. Stir together rice vinegar, sugar, and salt in a small bowl until salt is dissolved. Drizzle over rice; gently fold together. Lightly pack rice into a 1-cup dry measuring cup; invert onto prepared pan. Repeat with remaining rice, creating 2 rows of 3. Moisten hands slightly; gently press rice into an even layer. Place another piece of plastic wrap directly on surface of rice; press firmly into a compact, even layer (1/2 inch to 5/8 inch thick). Fold overhanging plastic wrap over top, gently pressing on top and smoothing outer edges. Chill 8 hours or overnight.
Preheat oven to 475°F with oven racks in middle and lower third of oven. Remove baking sheet from refrigerator. Unwrap plastic wrap, and remove top piece on rice; invert rice onto a work surface. Remove plastic wrap from back. Cut into about 56 (1 1/2- x 1-inch) pieces. (For clean slices, dip knife into warm water, and wipe clean often.) Lightly coat top of rice pieces with cooking spray. Brush 2 rimmed baking sheets evenly with oil. Place 28 rice pieces, cooking spray–coated sides down, on each oiled baking sheet. Bake in preheated oven until crisp and lightly golden, 14 to 20 minutes. Flip and top immediately.