1 cup firmly packed dark brown sugar
1 cup butter
1/3 cup heavy cream
4 cups powdered sugar, sifted
1 teaspoon vanilla extract
1/2 teaspoon sea salt
Bring first 2 ingredients to a rolling boil in a 3 1/2-qt. saucepan over medium heat, whisking constantly.
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.