Bring a large pot of salted water to a boil and cook campanelle to al dente according to package directions. Meanwhile, add butter to the meatball skillet and melt over medium heat. Once melted, whisk in flour and cook, whisking constantly, about 1 minute, or until the flour-butter mixture (called a roux) is light brown. Whisk in beef stock and cook, whisking constantly, until slightly thickened, 1 to 2 minutes. Stir in crème fraîche and season with salt and pepper. Add meatballs and cook until heated through and gravy has thickened, 8 to 10 minutes.