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Place butter in a mixing bowl or the bowl of a stand mixer.
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Turn on medium speed and start mixing butter until the tablespoons of butter are broken down and smooth.
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Turn mixer off, add 2 cups of powdered sugar to bowl. Mix on low until sugar is no longer visible. Turn mixer off again, scrape down sides.
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Add remaining powdered sugar, salt, vanilla (and food coloring if using) to bowl. Mix on low until sugar is incorporated and no longer visible.
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Add milk or cream while mixing on low.
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When all ingredients are in bowl, turn mixer to high and mix for about 2 minutes. Stop and scrape down sides a couple of times and then resume beating frosting.
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Add more milk if needed to spreading consistency.
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When cake is completely cooled, place on stand or plate. Frost bottom layer with about 1 cup of frosting, then add top layer of cake (I usually turn this layer upside down) so the top of the cake is facing the base layer, or dome side down. I rarely cut my cake to make it even!
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Frost the sides then top of the cake. Immediately top with any sprinkles or cake topper and refrigerate to set or serve immediately.