Line a 9-inch square pan with foil and grease with with 1-1/2 tsp butter; set aside. In a heavy saucepan, combine sugar, sour cream and remaining butter. Bring to a boil over medium heat, stirring constantly. Cook and stir until a candy thermometer reads 234° (soft-ball stage), about 5-1/2 minutes.
Remove from the heat. Stir in chocolate until melted. Stir in marshmallow creme until blended. Fold in apricots and walnuts. Pour into prepared pan. Cover and refrigerate overnight. Using foil, lift fudge out of pan. Discard foil; cut fudge into 1-in. squares.