In a small cup, stir the rice vinegar, sugar, and kosher salt until dissolved. Gently fold this mixture through the warm cooked sushi rice with a spatula, then let it cool to room temperature.

Toss the diced salmon with low-sodium soy sauce and toasted sesame oil in a medium glass bowl. Cover and refrigerate until ready to build the tacos.

Whisk the Japanese mayonnaise and sriracha together in a small dish until completely uniform. Transfer to a small squeeze bottle or piping bag if available.

Heat the frying oil in a deep skillet or heavy pot to 180 °C (179 °C). Whisk the tempura batter mix and ice-cold water together in a shallow plate until just combined.

Lightly dip only the rough side of each nori sheet into the tempura batter. Lower one sheet at a time into the hot oil, immediately using kitchen tongs to bend and hold it into a taco shell fold for 30 to 45 seconds until golden and rigid. Transfer to paper towels to drain.

Spoon two tablespoons of seasoned sushi rice into the base of each crispy nori taco shell. Top evenly with marinated salmon cubes, diced avocado, a generous drizzle of spicy sriracha mayo, sliced scallions, and toasted sesame seeds.