Add the shallot to the skillet and cook over moderate heat until softened, about 1 minute. Add the Calvados and cook, stirring, until reduced by half, 1 to 2 minutes. Add the chicken broth and apple cider and simmer until reduced to 1 cup, scraping up any browned bits from the bottom of the pan, 5 minutes. Add the vinegar and the remaining 2 tablespoons of ground gingersnaps and 1 tablespoon of butter and cook, stirring, until thickened, 1 minute. Return the pork to the skillet and cook for 1 minute longer. Transfer the pork to plates and spoon the sauce on top. Garnish with the crushed gingersnaps and chives.