Zest one lemon and set aside the zest. Juice two lemons and discard the pulp.
In a large saucepan over medium heat, cook the grappa (or wine) with the butter, lemon juice, and 1/2 of the zest. When reduced and slightly syrupy, add the cream, and salt and pepper to taste. Cook on low heat until further reduced, about 3-5 minutes.
Cook the spaghetti until al dente (about 2 minutes less than the package directions), drain and reserve 1 cup of the cooking water.
Add the pasta to the pan with the lemon cream sauce, and add some of the reserved pasta water if the sauce seems too dry. When mixed together, place the spaghetti in a warm serving bowl and toss in the Parmigiano.
Garnish with the remaining lemon zest, and serve immediately