Place warm cooked sushi rice into a broad bowl and gently fold in the seasoned rice vinegar using a slicing motion. Let the rice rest uncovered until it cools to lukewarm room temperature.

Toss the diced sushi-grade salmon with soy sauce and toasted sesame oil in a small bowl until evenly coated. Cover and chill in the refrigerator while preparing the taco shells.

Whisk the tempura batter mix and ice-cold water together in a shallow dish until just blended, leaving a few tiny lumps. In a separate small ramekin, mix Kewpie mayonnaise with sriracha to create the spicy drizzle.

Heat vegetable oil in a medium skillet to 350 degrees F (175 degrees C). Dip only the shiny outer side of one nori rectangle lightly into the tempura batter, then immediately lay it into the oil batter-side down. Using cooking tongs, promptly bend the sides upward to hold a taco shell curvature for 20 to 30 seconds until the batter turns golden and rigid. Transfer to a paper towel-lined wire rack and repeat with remaining nori pieces.

Fill each crispy nori shell with two tablespoons of seasoned sushi rice. Top generously with marinated salmon cubes and diced avocado. Drizzle with the spicy Kewpie mayo and finish with sliced green onions and toasted sesame seeds.