Use Kewpie mayonnaise if you can find it. Crush the sesame seeds using the bottom of a heavy skillet or in a spice grinder. For some added heat, serve with sriracha.
Whisk mayonnaise, toasted sesame seeds, vinegar, soy sauce, and sesame oil together in bowl. Bring 4 quarts water to boil in large pot. Stir in noodles and cook according to package directions, stirring occasionally, until noodles are cooked through but still retain some chew. Drain noodles and rinse under cold water until chilled. Drain well and transfer to large bowl. Add ⅓ cup dressing, snow peas, radishes, and scallions to noodles and toss to combine. Season with salt to taste.
Sprinkle salmon all over with salt, then sprinkle flesh sides with raw sesame seeds. Arrange salmon skin side down in cold 12-inch nonstick skillet and cook over medium-high heat until fat is rendered and skin is crispy, about 7 minutes. Flip salmon and continue to cook until center is still translucent when checked with tip of paring knife and registers 125 degrees (for medium-rare), about 7 minutes longer. Serve, drizzling salmon with extra dressing.
