Preheat oven to 350 degrees.

In a medium bowl, mix together 2/3 cup butter, white sugar and flour until evenly crumbly. Press into a 9 inch square baking pan lined with parchment paper. Bake for 20 minutes.

In a 2 quart saucepan, combine 1/2 cup butter, brown sugar, corn syrup and sweetened condensed milk. Bring to a boil, stirring constantly. Continue to boil for 5 minutes. Remove from heat and beat vigorously with a wooden spoon for 3 minutes. Pour over warm or cool baked crust. Cool until it begins to firm.

Place chocolate chips in a microwave safe bowl. Heat for 1 minute on high power, then stir. Continue to heat and stir at 20 second intervals until chocolate is melted and smooth. Pour chocolate over the caramel layer and spread evenly to cover completely. Sprinkle with sea salt and chill. Lift squares out of pan using parchment paper and place on cutting board. Cut into squares. Makes 36 squares.