Place rice in a fine mesh strainer, then run under cool water to thoroughly rinse. At first, the water will look cloudy or murky, the water should run clear. Cook the rice according to package or rice cooker instructions.
In a small saucepan, combine rice vinegar, sugar, and Kosher salt. Bring to a simmer over medium-high heat, then whisk until dissolved. Set aside until the rice is cooked.
Pour sushi vinegar on top of rice, directly in the pot it was cooked in. Use a rice spoon or spatula to gently toss the rice with the vinegar, being careful not to squish the rice.
Transfer the rice to a small pan or a small rectangular Tupperware lined with plastic wrap. Press the rice into a flat even layer. Cover with plastic wrap, then refrigerate overnight, or at least a few hours.
Cut the raw salmon into a small dice, then run a knife back and forth over it repeatedly until very finely chopped. Transfer the salmon to a large mixing bowl.
In a small bowl, combine Kewpie, Sriracha, and ponzu. Whisk well. Pour as much of the spicy mayo on top of the salmon as you like, then mix well to combine. Cover with plastic wrap, then refrigerate until needed.
Place spicy salmon on top of each crispy rice. Sprinkle with a pinch of sesame seeds, then top with a slice of jalapeño. Drizzle with a little additional spicy mayo, if wanted, then enjoy immediately!