1. In a 3-quart saucepan, combine light brown sugar and butter.
2. Place the pan on medium heat, stir constantly and bring it to a rolling boil.
3. Stir in cream and return just to a boil.
4. Remove the pan from heat and pour into the bowl of a stand mixer.
5. Gradually add confectioner's sugar and vanilla. Beat at medium speed using the whisk attachment after each addition, for 4-10 minutes or until thickened.
6. When the frosting is of spreading consistency but still hot, spread it on cake, and sprinkle salt on top. cut and serve.
The whisking time depends on the cooking of the syrup.