In a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed until smooth and creamy, 1-2 minutes. Add the cream cheese and continue beating until the mixture is completely smooth with no lumps, scraping down the bowl as needed.
Pour in the cooled salted caramel sauce and vanilla. Beat on medium speed until fully combined and silky. If the mixture looks loose or warm, chill it for 5-10 minutes before adding sugar.
With the mixer on low speed, gradually add 3 cups of the powdered sugar, about 1/2 cup at a time, mixing until just combined after each addition. Add the salt, then increase the speed to medium and beat until the frosting looks light and fluffy, 1-2 minutes.
Check the consistency: For spreading on cakes, it should be thick yet easily spreadable; for piping, add up to 1 more cup powdered sugar, a few tablespoons at a time, beating just until it holds medium peaks. If it becomes too soft at any point, refrigerate for 10–15 minutes to firm up.
Use immediately or cover and refrigerate. Makes about 4-5 cups of frosting, enough to generously frost a 9x13-inch cake, a 2-layer 8- or 9-inch cake, or about 24 cupcakes.