1. Bring sugar, water, and fresh lemon juice (to prevent crystals from forming) to a boil in a tall, heavy saucepan over medium-high heat; boil 3 minutes or until sugar melts and liquid is clear, swirling pan occasionally. Cover; boil 1 minute.
2. Remove lid. Boil, gently swirling often and checking color and temp every 5 to 10 seconds, about 4 minutes or until a candy thermometer reaches 345° to 350° and mixture is medium to dark amber. Remove from heat.
3. Gradually whisk in cream. (Mixture will bubble and spatter.) Cook, whisking constantly, over low heat 1 minute or until smooth. Remove from heat; stir in vanilla and sea salt. Transfer to a serving bowl or pitcher. Chill, covered, up to 1 week.