Butter or lightly spray with non-stick spray a 10 x 15-inch glass cake pan; set aside.
In a large mixing bowl add chocolate chips, butter, vanilla and pecans; set aside.
In a heavy duty sauce pan combine white granulated Pioneer Sugar, marshmallows and evaporated milk. Stir to mix ingredients and cook over medium-high heat for 8 minutes, stirring constantly.
Reduce heat to low and continue to cook for an additional 4 minutes, stirring constantly. Mixture will appear foamy and cream-colored.
Pour the hot mixture into the bowl with the other ingredients and beat until smooth and all the butter and chips are melted.
Spread fudge mixture into prepared pan. Lightly cover with saran wrap or wax paper and chill for several hours until firm. Cut into small pieces.