Add the shallots to the skillet and cook, stirring often, until golden, about 2 minutes. Gradually add the Madeira and bring to a boil, stirring; boil until reduced to 3/4 cup. Return the pork to the skillet and simmer, turning twice, until cooked and the sauce is slightly thickened, about 3 minutes. Arrange the pork on plates. Swirl the remaining 1 tablespoon of butter into the sauce and season with salt and pepper. Spoon the sauce over the pork and serve.