Preheat oven to 350°F.
Combine flour, ½ cup butter and 3 Tablespoons sugar in a food processor or medium bowl. Pulse or mix with a pastry blender until dough resembles coarse crumbs. Press into the bottom of a foil-lined 9-inch square pan.
Bake until lightly browned, about 20 minutes. Remove from oven and sprinkle with chocolate chips; let stand 2 minutes, then spread chocolate over crust.
Place 1 1/3 cups sugar in a large heavy saucepan; stir over low heat until melted and caramel colored, about 20 minutes. Stir in 1 cup butter, cream and corn syrup. Mixture will bubble up and clump together. Simmer over medium-low heat stirring frequently until mixture is smooth and reaches 234 to 240°F (soft ball stage) on a candy thermometer, about 35 minutes.
Remove from heat and stir in walnuts. Immediately pour over crust. Drizzle with melted chocolate. Refrigerate until firm.
Use foil to lift out of pan; transfer to cutting board. Cut into 16 squares.
Note: To test for soft ball stage without a thermometer, drop a small amount of sugar mixture into a dish of very cold water. If sugar has reached soft ball stage, it will form a soft ball that can be flattened and picked up out of the water.