Line a 13x9x2-inch baking pan with foil, extending foil over edges of pan. Butter foil; set pan aside.
Butter the sides of a heavy 3-quart saucepan. In the saucepan, combine the sugar, evaporated milk, and the 1 cup butter. Cook over medium-high heat to boiling, stirring constantly with a wooden spoon to dissolve sugar (about 7 minutes). Avoid splashing mixture on side of pan. Carefully clip candy thermometer to side of pan.
Cook over medium heat, stirring frequently, until thermometer registers 236 degree F (soft-ball stage). Mixture should boil at a moderate, steady rate over the entire surface. Reaching soft-ball stage should take about 12 minutes.
Remove saucepan from heat; remove candy thermometer from saucepan. Add white chocolate, marshmallow creme, chopped pecans, and vanilla. Stir until well blended and the chocolate is melted. Fold in semisweet chocolate pieces (don't stir until melted). Stir to marble; quickly turn into prepared pan.
While fudge is warm, score it into 1-inch squares. When candy is firm, use foil to lift it out of pan; cut into squares. Store tightly covered in the refrigerator. Makes 117 pieces or about 3-1/2 pounds.