Place corned beef and contents of seasoning packet in a Dutch oven; cover with water. Add onion and celery. Bring to a boil. Reduce heat; cover and simmer for 2-1/2 hours or until meat is tender. Drain and discard liquid and vegetables. Place beef on a rack in a shallow roasting pan; set aside.
In a saucepan, melt butter over medium heat. Stir in the remaining ingredients. Cook and stir until sugar is dissolved. Brush over beef. Bake, uncovered, at 350° for 25 minutes. Let stand for 10 minutes before slicing.
TEST KITCHEN TIP Slice corned beef against the grain for the most tender slices of meat.