A great treat for holiday catering.
Line an 8- or 9-inch square pan with foil (leave a 1-inch overhang on 2 sides) and spray with cooking spray.
Combine corn syrup, 8 fluid ounces cream, milk, sugar and 1/4 teaspoon sea salt in a 4-quart saucepan. Bring to boil over medium heat, stirring occasionally.
Reduce heat to medium-low and continue cooking (at a low boil) and stirring occasionally until temperature on candy thermometer reaches 234°F or small amount of mixture dropped into very cold water forms a soft ball which flattens on removal. This will take about 40 minutes.
Gradually add remaining 8 fluid ounces in a thin steady stream, stirring constantly.
Continue cooking over medium-low heat, stirring frequently until temperature on candy thermometer reaches 244°F or small amount of mixture dropped into very cold water forms a firm ball which does not flatten on removal. This will take about 30 minutes. Remove from heat.
Stir in vanilla. Pour into prepared pan. (NOTE: Do NOT scrape side of saucepan.) Place on wire rack and cool 1 hour. Sprinkle top evenly with remaining 1/2 teaspoon sea salt.
Lift cooled candy from pan using foil. Cut into 1-inch squares. Wrap squares individually in waxed paper or plastic wrap. Store in tightly-covered container.