Combine chocolate morsels, marsh-mallow creme, and butter in a large bowl.
Combine sugar and evaporated milk in a heavy saucepan; bring to a boil, reduce heat, and cook over low heat 9 minutes, stirring constantly.
Pour over chocolate morsel mixture; stir until chocolate melts and mixture is smooth.
Add pecans; stir until well blended.
Spread mixture in a lightly buttered 15- x 10- x 1-inch jellyroll pan.
Chill until firm; cut into squares.
Cover and store in refrigerator.