Start by preparing your mixing equipment. If you have a stand mixer, attach the whisk or paddle attachment. If not, a handheld electric mixer will work just fine. Ensure that the butter is at room temperature for easy whipping.
Place the unsalted butter in the bowl of your stand mixer or a mixing bowl if using a handheld electric mixer. Beat the butter on high speed for approximately 5 minutes or until it becomes lightened in color and has a whipped consistency. Remember to occasionally scrape down the bowl's sides to ensure even mixing.
While the butter is whipped, sift 4 1/2 to 5 cups of powdered sugar. Sifting helps remove any lumps and ensures a smooth and velvety frosting. Reduce the mixer speed to medium-low and gradually add powdered sugar, about one heaping tablespoon at a time. Allow each addition to fully incorporate before adding more. The amount of powdered sugar can be adjusted to your desired level of sweetness.
Once the powdered sugar is fully incorporated, it's time to enhance the flavor. Add 1/4 teaspoon of fine salt, or adjust to taste. The salt helps balance the sweetness and brings out the flavors in the frosting. Follow this with four teaspoons of vanilla and continue beating until the ingredients are well incorporated. To achieve the perfect consistency, add three tablespoons of heavy cream or whole milk, one tablespoon at a time. Start with three tablespoons and add more if needed to reach your desired texture. The cream or milk adds moisture and creaminess to the frosting. Continue beating for approximately one more minute to ensure a whipped and well-combined frosting.