Mix sugar, water and corn syrup in a small saucepan.
Bring the water and sugar mixture to boil over medium heat, but DO NOT STIR. (Stirring encourages crystallization). Let the mixture cook until it caramelizes into a golden brown color. It happens quickly, so keep an eye on it.
Remove the caramelized sugar syrup from the heat and slowly mix in the cream. Pour in just a tiny bit at a time, especially at the beginning, and mix to combine. You can start adding more and more as you go along.
Mix in salt and vanilla. Stir to combine.
Let cool to room temperature before using in buttercream or adding in between layers.