The day before, make the sushi rice. Place rice in a fine mesh strainer, then run under cool water to thoroughly rinse. At first, the water will look cloudy or murky, but after 1 minute, the water should run clear. Cook the rice according to package or rice cooker instructions.
Make the sushi vinegar. In a small saucepan, combine 2 Tbsp rice vinegar, 1½ tsp sugar, and ½ tsp Kosher salt. Bring to a simmer over medium-high heat, then whisk until dissolved. Set aside until the rice is cooked.
Season the rice. 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.
Cool overnight. Transfer the rice to a small ¼ sheet pan or a small rectangular Tupperware lined with plastic wrap. Press the rice into a flat even layer, about ½” tall. Cover with plastic wrap, then refrigerate overnight, or at least a few hours. (You can freeze for 2 hours if you want to speed up the process.)
Finely chop the salmon. 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. To this, add 3 thinly sliced scallions.
Make the spicy sauce. In a small bowl, combine ½ cup Kewpie, ¼ cup Sriracha, and 3 Tbsp ponzu. Whisk well. Pour as much of the spicy mayo on top of the salmon as you like (reserve the rest for serving!), then mix well to combine. Cover with plastic wrap, then refrigerate until needed.
Make the crispy rice. Heat ½ cup of neutral oil over medium heat in a large nonstick skillet with enough to coat the bottom with a thin layer. While the oil heats, use a sharp knife to cut the rice into bite-size rectangles about 2-inches by ¾-inch, making sure the rice is compact. It can be helpful to lightly wet your hands with water if the rice is too sticky. When the oil is hot, add 7 or 8 pieces of rice, making sure not to crowd the pan. Cook for ~2 minutes per side, or until the rice is a light golden brown. Carefully transfer the crispy rice to a cooling rack lined with a paper towel. Repeat with remaining rice, adding more oil between batches, if needed.
Assemble and serve! Place about 1 heaping tablespoon of 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!