Preheat oven to 350 degrees F (175 C).
In a medium bowl, mix together ⅔ cup butter, white sugar, and flour until evenly crumbly. Press into a 9-inch square baking pan. Bake for 20 minutes.
In a 2 quart saucepan, combine ½ cup butter, brown sugar, corn syrup, and sweetened condensed milk. Bring to a boil. Continue to boil for 5 minutes. Remove from heat and beat vigorously with a wooden spoon for about 3 minutes. Pour over baked crust (warm or cool). Cool until it begins to firm.
Place chocolate in a microwave-safe bowl. Heat for 1 minute, then stir and continue to heat and stir in 20-second intervals until chocolate is melted and smooth. Pour chocolate over the caramel layer and spread evenly to cover completely. Chill. Cut into 1-inch squares. These need to be small because they are so rich.
