Place salmon in a 13- x 9-inch baking dish. Combine kosher salt, sugar, and wasabi powder. Brush salmon with mirin, and top with salt mixture. Cover fish with plastic wrap.
Nest another dish of the same size on fish, and set 2- to 3-pound weights (such as unopened canned goods) in top dish. Place in the refrigerator at a slight incline so juice formed during curing process drains away. Cure 2 to 4 days.
Scrape off salt mixture. Place salmon, skin side down, on a board; cut fillet into thin diagonal slices, and remove fish from skin, discarding skin. Serve cured salmon on pumpernickel bread with Wasabi Yogurt.