Add butter, syrup, water and salt to cider in a pan. Simmer gently 5-6 minutes. Let cool slightly. Whisk in beaten egg yolks. Beat whites separately till stiff peaks form. Gently fold in cider mix until evenly blended. Pour into prepared pie shell. Dust with nutmeg. Bake at 350F, 30-35 minutes until filling is well-set.