Incorporate powdered sugar 1/2 cup at a time, waiting until combined before adding more.
Once all powdered sugar is mixed into butter, add salt and vanilla extract.
Add the whipping cream in a slow steady stream with the mixer still on.
When making buttercream, it is imperative to scrape the sides of the mixer consistently, as this helps to incorporate everything evenly. Particularly if you have a mixer like mine, with a deep bowl, it is important to scrape the bottom to make sure everything is mixed in evenly.Adapted from a Preppy Kitchen recipe.