Cook the sushi rice according to package instructions. While rice cooks, combine rice vinegar and sugar in a microwave-safe bowl and microwave for 30 seconds until sugar dissolves.
Pour the vinegar mixture over the cooked rice and fold it in gently. Set aside while you prepare the salmon.
Preheat your oven to 400°F.
Pat the salmon fillet dry with paper towels, then cut into bite-sized cubes. In a bowl, combine the cubed salmon with soy sauce, Kewpie mayo, sriracha, and sesame oil. Mix until well coated.
Cut each nori sheet into 4 equal squares to make 12 squares total.
Spray a 12-cup muffin tin generously with cooking spray. Place one nori square in each cup, pressing it down to form a bowl shape.
Add a generous spoonful of seasoned sushi rice onto each nori square. Use the back of a spoon or your fingers to press the rice down firmly, creating a compact, even base.
Top each rice layer with a heaping spoonful of the salmon mixture, pressing down gently so everything holds together.
Bake in the preheated oven for 15 minutes, or until the salmon is cooked through and the edges of the nori are crispy and slightly golden.
Remove from oven and let the cups cool for 5 minutes. Use a butter knife to gently loosen the edges, then pop each cup out onto a serving platter.
Drizzle with more sriracha or spicy mayo, sprinkle with furikake, and scatter sliced green onions on top. Serve immediately while warm.