Place the softened butter in the bowl of a stand mixer fitted with the paddle attachment. Beat on medium-high speed for about 5 minutes, stopping several times to scrape the sides and bottom of the bowl. The butter should become smooth, creamy, and noticeably lighter in color.
Add the powdered sugar, vanilla, salt if using, and 1 tablespoon of whole milk. Start the mixer on low speed so the powdered sugar stays in the bowl. Mix until the ingredients are combined.
The frosting should be thick, smooth, and able to hold its shape. If it feels too stiff, add the remaining milk one teaspoon at a time until you reach the consistency you want.
For an extra-smooth buttercream, keep the head of an immersion blender completely submerged in the frosting and slowly work it through the buttercream until the larger air bubbles disappear.
Use the frosting right away for cupcakes, mini cakes, layer cakes, or sheet cakes, or transfer it to an airtight container for storage.