Preheat your oven to 375°F.
Rinse the sushi rice under cold water until the water runs clear. Combine the rice and water in a medium saucepan and bring to a boil. Once boiling, reduce the heat to low, cover, and simmer for 18-20 minutes until the rice is tender and the water is absorbed.
Remove the rice from heat and let it sit, covered, for 10 minutes. In a small bowl, mix the rice vinegar, sugar, and salt until dissolved. Gently fold this mixture into the cooked rice and let it cool slightly.
In a separate bowl, combine the spicy salmon, mayonnaise, sriracha, green onions, and cucumber. Mix until well combined.
Grease a muffin tin with cooking spray or oil. Press a spoonful of sushi rice into the bottom of each muffin cup, creating a small base. Add a spoonful of the salmon mixture on top of the rice, then cover with more rice, pressing down gently to pack it in.
Sprinkle nori pieces on top of each muffin and add sesame seeds if using. Bake in the preheated oven for 15-20 minutes, or until the tops are golden and crispy.
Allow the muffins to cool for a few minutes before removing them from the tin. Serve warm or at room temperature.