Place very soft butter and a very soft cream cheese into the bottom of a large bowl.: The first sensory cue you notice is the gentle give when you press the softened butter and the slight tack of the cream cheese . At this stage the aroma is dairy forward and cool. Mixing two soft fats together ensures an even emulsion later, which prevents grainy pockets of sugar. A common mistake is working with cold ingredients, which results in uneven texture and small clumps. Use a rubber spatula to press them together in the bowl so the mixer can grab the mass easily, and pause to check the temperature by touching the sides of the bowl. If they feel too firm, let them sit five to ten minutes until they yield to gentle pressure.
On top of that, pour powdered sugar and vanilla extract.: When the powdered sugar hits the bowl it creates a faint cloud of sweetness and powdery aroma, and adding the vanilla extract introduces a fragrant, warm note. Adding dry and liquid elements at once encourages the powdered sugar to absorb moisture gradually, but you should add slowly if you are worried about dust. A mistake to avoid is dumping all the sugar at once which can create a dusty mess and uneven incorporation. Measure carefully and, if your powdered sugar is clumpy, sift it first so the mixture remains smooth.
Use an electric mixer or stand mixer with the whisk attachment and mix it all together on low speed until completely smooth and thick. At first, it might seem like the powdered sugar won't all mix in, but as you stir and work it together with the hand mixer, it will surely come together.: You will hear a steady whir and see the ingredients begin to coalesce into a pale, velvety mass. The sound changes from separate particles being moved to a cohesive, muffled hum as the mixture becomes uniform. This low speed prevents the powdered sugar from spitting out of the bowl and keeps the emulsion stable. The goal is a thick, lump free texture; if you rush to high speed too early you can powder the room and create air bubbles that collapse later. If the mixture seems resistant at first, be patient and let the mixer work, occasional pauses to scrape ensure even mixing.
Scrape the sides of the bowl, increase to medium speed if desired and mix again briefly to make sure it's fully smooth.: When you scrape, you ll see pockets of unmixed powdered sugar or streaks of cream cheese that need folding in. After scraping the surface should look glossy and consistent, and increasing speed for a short burst incorporates air for a lighter texture. You can feel the frosting become silkier under the beaters, and visually it will lose any grainy appearance. A common problem is overmixing at high speed which makes the frosting too soft and aerated. Limit the medium speed burst to brief pulses, watching for the glossy, ribbon like fall from the whisk.
Use as desired.: At this point the frosting should spread easily, smelling sweet with a pleasant tang from the cream cheese . The texture should hold peaks slightly, ideal for both spreading and piping. If it seems too soft, chill briefly to firm it up; if too stiff, let it sit at room temperature and stir gently. Avoid applying hot or warm frosting to a delicate cake as it may slide off. For best results work quickly but gently, and store any leftovers in an airtight container in the fridge.