Bring a large pot of lightly salted water to a boil. Cook spaghetti in the boiling water, stirring occasionally until tender yet firm to the bite, about 12 minutes. Drain; return to the pot.

Melt butter in a sauté pan over medium heat. Add leek; cook and stir until slightly softened, 6 to 7 minutes. Season with salt; add white wine and lemon juice and bring to a boil while scraping the browned bits of food from the bottom of the pan with a wooden spoon. Simmer until most liquid evaporated, about 5 minutes.

Stir crème fraîche, mustard, and 1 pinch cayenne pepper into leek mixture; reduce heat to low and cook until warmed, about 5 minutes. Add salmon to sauté pan; cook and stir until salmon starts to flake and is no longer pink in centers, 2 to 3 minutes. Off heat, stir in cilantro. Pour salmon sauce over spaghetti; toss to coat. Season with remaining 1 pinch cayenne pepper.