Butter 13x9-inch pan; line with parchment or waxed paper. Butter paper.
Sprinkle 3/4 cup milk chocolate chips, 3/4 cup semi-sweet chocolate chips, 2 tablespoons almonds, 2 tablespoons pecans and 2 tablespoons walnuts over parchment paper; set aside.
Combine sugar, butter, water and salt in heavy 4-quart saucepan. Cook
over medium heat, stirring constantly, until mixture comes to a full boil (10
to 12 minutes). Continue boiling, stirring often, until candy thermometer
reaches 305°F or small amount of mixture dropped into ice water forms a
hard brittle strand (20 to 25 minutes).
Remove from heat; stir in vanilla.
Pour mixture into prepared pan.Immediately sprinkle with remaining almonds, pecans and walnuts; swirl into toffee with knife. Sprinkle remaining milk chocolate chips and semi-sweet chocolate chips evenly over toffee, pressing gently into toffee.