Combine all ingredients in stand mixer fitted with whisk attachment. Start on low to incorporate confectioners' sugar, then gradually increase to high.
Whip on high speed for 5 minutes, until very pale and very fluffy. This long whipping time is critical for the frosting to be light, satiny, mousse-y and like the most delicious, beautiful-textured vanilla frosting ever.
Pause several times to scrape down sides of mixer bowl while it is beating. The frosting will double in volume and will go from a dull, matte white to a shiny, satiny, fluffy white when it is ready.
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Notes
VERY IMPORTANT: You must allow the frosting to whip for a solid 5 minutes. Letting the mixer run for a prolonged time allows the butter and sugar to whip up into a soft, airy texture.Air is an ingredient. Whipping for 5 minutes allows you to make a big volume of frosting without adding tons of extra icing sugar. If it is still dense and fudgy, it's not done yet. It will be soft and shiny when you're done beating.Hand Mixer Method: You can use a basic electric hand mixer instead of a stand mixer but you'll have to beat for longer since it has less power. About 6–7 minutes on high with a hand mixer equals 5 minutes on a stand mixer. Just watch for the frosting to turn pale, fluffy, and satiny.This frosting is perfectly sweet, never chalky and oversweet, thanks to the higher ratio of butter to sugar combined with the whipping technique. It tastes and acts more like a fancy Italian or French buttercream than the simple American buttercream that it is.Butter: ONLY use unsalted, no room for negotiation. If you use salted butter, this will be inedibly salty. Please trust me on the quantities and you will not be disappointed.This makes enough frosting for 24 simply-frosted cupcakes, one 9x13" slab cake, or two 9" cake layers. If you want enough frosting to pipe details on the cake or make dramatic cupcake swirls, make 1.5 x the recipe.If you'd like to add food coloring, add it by a few drops at a time at the end and beat just until combined. Start with a little. You can always add more but you can't take it away.You can add a splash more milk if you like to reach the desired consistency. Note that it will continue to thicken up as the sugar hydrates.