Combine rice vinegar, sugar, and salt in a small bowl, stirring until completely dissolved. Pour over freshly cooked, still-warm sushi rice and fold gently using a cutting motion to avoid mashing the grains.

Transfer rice to a plastic wrap-lined baking pan (approximately 8x8 inches). Press firmly into an even, compact layer about 1 inch thick. Cover tightly with plastic wrap and refrigerate for at least 4 hours or overnight for best results.

Dice sushi-grade salmon into small, uniform pieces about 1/4 inch in size. Combine with Kewpie mayo, sriracha, soy sauce, scallions, and sesame oil. Mix thoroughly and refrigerate in a covered bowl until ready to use.

Remove chilled rice from refrigerator and cut into 16 equal rectangles (4 rows by 4 rows). Use a sharp knife dipped in water between cuts to prevent sticking.

Heat about 2 to 3 tablespoons vegetable oil in a large skillet over medium heat. Test oil temperature by dropping in one grain of rice; if it sizzles immediately and vigorously, oil is ready.

Fry 4 to 6 rice rectangles at a time for 3 to 4 minutes per side until golden brown and crispy. Avoid overcrowding the pan. Transfer to paper towel-lined plate. Repeat with remaining rice, adding fresh oil between batches.

Top each crispy rice cake with a slice of avocado, 1 tablespoon spicy salmon mixture, and a jalapeño slice. Sprinkle with toasted sesame seeds. Serve immediately while rice is warm and crispy. Assemble no more than 5 minutes before serving.