Combine the wasabi powder, rice vinegar and water in a small bowl. Let rest for 1 hour.
In a large stockpot, place the potatoes and a generous pinch of salt. Cover with cold water and bring to a simmer. Cook at a simmer until the potatoes are fork-tender, about 15 minutes. Drain the potatoes and return to the pot.
Add the butter, milk, buttermilk, salt and pepper. Mash until smooth. Fold in the wasabi paste.
Meanwhile, preheat the oven to 400°F.
Pat the salmon fillets dry and season liberally with salt and pepper.
Place on a sheet pan and roast until cooked through, 12-15 minutes.
In a medium saucepan, heat the soy sauce, ½ cup water, sugar, scallions, garlic, sake, dried shiitakes, and ginger. When the mixture reaches a boil, use a strainer to remove the vegetables. Simmer for 20 minutes.
In a small bowl, mix together the cornstarch and 1 tablespoon water. Add the slurry to the sauce and return to a boil. Remove from heat.
To make the sour cream cucumbers, combine the dill, sour cream, vinegar, salt and pepper in a bowl. Fold in the cucumbers.
To serve, divide the mashed potatoes between plates. Top with salmon, drizzle with teriyaki sauce, and serve with sour cream cucumbers on the side. Sprinkle with sesame seeds.