In a large pot of boiling salted water, cook the spaghetti until al dente.
Meanwhile, in a large, deep skillet, melt the butter. Add the lemon zest and cook over moderate heat, stirring, for 1 minute. Add the cream and bring to a boil. Simmer over moderate heat for 3 minutes. Add a pinch each of salt and pepper and remove from the heat. Stir in the ricotta.
Drain the pasta. Add it to the skillet and toss well. Add the Parmesan, season with salt and pepper and toss again. Transfer to bowls and serve.