In a straight-sided skillet over medium-low heat, bring 2 inches of water and a pinch of salt to a boil. Gently place salmon in water. Poach at a low simmer until cooked through, about 7 to 8 minutes. Remove and let cool; flake into large pieces.
Meanwhile, bring a large pot of salted water to a boil. Add soba and cook until al dente. Drain and rinse under cold water.
In a large bowl, toss soba with buttermilk, olive oil, lemon juice and zest, chives, and salt and pepper to taste. Add watercress and salmon, and gently combine. Serve or refrigerate until ready to eat.