Preheat oven to 350°. In a large bowl, beat butter until fluffy. Add the flour, sugar and salt; beat just until crumbly. Set aside 1 cup for topping. Press remaining crumb mixture into a greased 13x9-in. baking pan. Bake 10-12 minutes or until set and edges begin to brown.
Meanwhile, in a small saucepan, combine milk and 1-1/2 cups semisweet chocolate chips. Cook and stir until chips are melted. Remove from the heat; stir in vanilla. Spread mixture over crust.
Combine the nuts and milk chocolate chips with the remaining semisweet chocolate chips and crumb mixture. Sprinkle over filling. Bake 15-20 minutes or until center is set. Cool on a wire rack. Cut into bars.
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