Preheat oven to 350 degrees F. Grease 8-inch-square baking pan. Combine flour, cocoa and salt in a small bowl. Beat butter, granulated sugar and brown sugar in a large bowl until creamy. Add 2 eggs, one at a time, beating well after each addition. Add vanilla extract; mix well. Gradually beat in flour mixture. Reserve 3/4 cup batter. Spread remaining batter into baking pan. Sprinkle nuts and white chocolate chips over batter. Drizzle caramel topping over top. Beat remaining egg and reserved batter in same large bowl until light in color. Stir in semi-sweet chocolate chips. Spread evenly over caramel topping. Bake 30-35 minutes or until center is set. Cool completely in pan on wire rack. Cut into squares.