Combine the buttermilk and sour cream in a small bowl and stir well. Park in the refrigerator until you’re ready to use them.
Place the butter and shortening in the work bowl of your favorite stand mixer and sift in the sugar. Install the paddle attachment and beat the mixture on low to incorporate. Add the vanilla and salt and increase the speed to medium. Continue to beat until light and airy — 5 to 6 minutes. Scrape down the sides and bottom of the bowl as needed.
Add the buttermilk-sour cream mixture and beat on low until integrated. Stop and scrape the bowl, then return the speed to medium until smooth — 15 seconds. Use immediately or tightly seal and refrigerate for up to 3 days or freeze for up to 1 month. If frozen, thaw overnight in the refrigerator prior to use. The frosting is fairly spreadable straight from the refrigerator, but it is even easier to use at room temperature.