1. Cook pasta in 6-8 quarts of rapidly boiling salted water until al dente (about 8-10 minutes).
2. Meanwhile, in top pan of double boiler placed over simmering water, melt the butter. Stir in walnuts, lemon zest and cognac and keep warm over low heat.
3. Drain pasta, place on serving platter, add walnut-butter sauce, sprinkle with salt and pepper to taste, then serve immediately.