Butter 9 x 13-in. (3-L) metal baking pan. Set aside. In large heavy saucepan over medium heat, mix both sugars, corn syrup, 1/2 cup (125 mL) water and all but 1 tsp (5 mL) whisky (reserve the teaspoon of whisky), stirring until sugar is melted.
Cover and increase heat to medium. Bring to a boil, without stirring, until a candy thermometer inserted reaches soft crack stage (270°F/132°C). Add butter and 1 tsp (5 mL) sea salt. Stir continuously until mixture reaches hard crack stage (300°F/149°C).
Remove saucepan from heat. Stir in vanilla and reserved 1 tsp (5 mL) whisky. Pour into prepared pan. (Do not scrape or spread.) Sprinkle with 3/4 tsp (4 mL) flaked sea salt. Cool completely. Hit candy on hard surface to break into pieces. Layer between pieces of waxed paper in airtight container for up to 2 weeks.
If you don't have a candy thermometer, test candy doneness by dropping a teaspoonful of the hot syrup into a cup of cold water. At the hard crack stage the syrup will form hard brittle threads.