Aunt Georgia's Fudge

3-1/3 C. Sugar
1 C. butter or margarine
1 C. packed dark brown sugar
1 can (12 ounces) evaporated milk
32 large marshmellows, halved
2 C. (12 ounces) semisweet chocolate chips
2 milk chocolate candy bars (7 ounces each), broken
2 squares (1 ounce each) semisweet baking chocolate, chopped
1 tsp. vanilla extract
2 C. chopped pecans

In a large saucepan, combine first four ingredients. Cook and stir over medium heat until sugar is disolved. Bring to a rapid boil; boil for 5 minutes, stiring constantly. Remove from the heat; stir in marshmellows until melted. Add chocolate bars and baking chocolate; stir until melted. Fold in vanilla and pecans; mix well. Pour into a greased 15-in. x 10-in. x 1-in. baking pan. Chill until firm. Cut into squares. Yield 5-1/2 pounds.