Preheat oven to 450°. Soak pecan halves in water to cover 20 minutes; drain well. Sprinkle 2 Tbsp. salt evenly over the bottom of a 15- x 10-inch jelly-roll pan. Arrange pecans in a single layer in pan; sprinkle evenly with 2 more Tbsp. salt. Place pecans in hot oven, and turn off oven. Let stand in oven 1 hour and 30 minutes. Toss pecans in a strainer to remove excess salt. Coarsely chop pecans, and cool.
Bring sugar, evaporated milk, and butter to a boil in a large heavy saucepan over medium heat; boil, stirring constantly, until a candy thermometer registers 234° (about 7 minutes).
Remove from heat; stir in marshmallows and chocolate morsels until smooth. Stir in 2 tsp. vanilla and chopped pecans.
Pour into a buttered 8-inch square pan; cool completely. Cut into 1-inch squares.