How to Make It
Bring a large pot of salted water to a boil. In a large, deep skillet, melt 1 tablespoon of the butter. Add the pancetta and cook over moderately low heat until most of the fat has been rendered, about 12 minutes. Increase the heat to moderate and cook, stirring occasionally, until the pancetta is golden brown, about 8 minutes. Add the shallots and sage and cook over low heat, stirring occasionally, until softened, about 8 minutes. Add the vin santo and simmer over moderate heat for 4 minutes, then stir in the chopped chestnuts and the remaining 1 tablespoon of butter.
Meanwhile, cook the spaghetti until al dente. Drain the pasta, reserving 1 cup of the cooking water. Add the spaghetti and the reserved cooking water to the skillet, season with salt and pepper and toss over low heat. Serve at once.