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1
Bring a large pot of well-salted water to a rolling boil. Cook the fresh egg tagliatelle until just al dente. Reserve a small measure of pasta water — no more than two tablespoons — before draining. The pasta should be ready to dress immediately; it must not sit.
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2
While the pasta cooks, melt the mountain butter in a wide pan over medium heat. Continue cooking, swirling the pan, until the butter passes through foam and reaches a light hazelnut colour with a distinctly nutty aroma. This is the beurre noisette stage. Pale butter lacks the depth the dish requires; butter that darkens beyond hazelnut is discarded and begun again.
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3
Remove the pan from the heat entirely. Add the drained tagliatelle and toss to coat in the brown butter, adding the reserved pasta water only if needed to achieve a light, even coating — not a sauce. The dish lives or dies here: the prosciutto goes in now, off all heat, and is folded through the pasta with the thyme or marjoram leaves. Thirty seconds of residual heat is sufficient to barely warm the prosciutto through. Any further cooking toughens the meat and destroys its sweetness.
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4
Divide immediately among four warmed plates. The prosciutto should lie in loose folds among the pasta, its fat translucent and glistening, its colour unchanged from raw. Serve at once.