1. Bring first 2 ingredients to a rolling boil in a 3 1/2-qt. saucepan over medium heat, whisking constantly.
2. Stir in cream, and return to a boil; remove from heat. Pour into bowl of a heavy-duty electric stand mixer. Gradually beat in powdered sugar and vanilla at medium speed, using whisk attachment; beat 7 to 10 minutes or until thickened. Use immediately. Sprinkle frosted cake with sea salt.