With poultry shears or a knife, cut each hen in half lengthwise through breastbone and backbone. Rinse and pat dry. Reserve giblets for other uses.
Set halves, skin side up, in a 9- by 13-inch pan. Brush skin with melted butter, then sprinkle with the shallots.
Bake in a 425° oven until meat at thigh bone is no longer pink (cut to test), about 40 minutes (35 minutes in a convection oven). Transfer hens to a platter; keep warm.
Spoon off and discard fat from pan drippings. Add calvados, broth, and tarragon to drippings. Set pan over medium-high heat and stir to scrape brown particles free.
Whisk in crème fraîche. Turn heat to high and boil until mixture is reduced to 3/4 cup, 5 to 8 minutes. Pour sauce into a bowl. Garnish hens with tarragon sprigs and add salt and pepper to taste. Serve with sauce.