In the bowl of a stand mixer fitted with a paddle attachment, or in a large bowl with a hand-held mixer, mix all of the ingredients on low speed until the frosting is combined (this helps prevent the powdered sugar from flying everywhere).
Increase the speed to high and beat for 4-5 minutes until frosting is fluffy, pausing once or twice to scrape down the sides of the bowl as needed. You'll notice the frosting become lighter in color, fluffier, and more airy as you whip.
(Optional) The frosting will be very airy with lots of bubbles at this point. To achieve a smooth frosting, beat it with a wooden spoon or spatula by hand for 1-2 minutes. This will push all of those air bubbles out.
Use frosting as desired to cover cakes, cupcakes, cookies, etc. It will form a "crust" as it sits, so cover the bowl with plastic wrap or put it in an airtight container to keep it fresh if not using immediately.
Store frosting in an airtight container in the refrigerator for up to 1 week or freeze for up to 1 month. Bring it back to room temperature, then beat for 1–2 minutes until light and fluffy before using.
Notes
Powdered sugar: If your sugar is excessively clumpy, sift after measuring to ensure frosting is smooth.
Salted vs. unsalted butter: I used to use unsalted butter in this recipe, but now prefer to use salted butter for additional flavor. Use what you like, and adjust the amount of salt according to your preferences.
Frosting amount: A lot depends on how well you beat the frosting, and what you plan to do with it. This recipe yields 4-5 cups of frosting, enough for a standard two-layer cake, 24 cupcakes and 36 sugar cookies. If you like to apply a generous amount of frosting, want to pipe a lot of decorations, or have a bigger project, double the recipe.
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