Cut the cold butter into pieces and beat on medium-high speed until smooth and fluffy. This takes about 5 minutes. Don’t rush it—let the butter fluff itself up.
Scrape down the sides and add 2 cups of powdered sugar. Mix on low speed to combine, then increase to medium-high and beat until fully incorporated.
Add the remaining powdered sugar, vanilla extract, and a tablespoon of heavy cream. Beat again until light and fluffy. Add more cream as needed for texture.
Toss in a pinch of salt and mix until the frosting tastes like a sweet cloud of vanilla goodness.
Pipe or spread immediately, or store covered in the fridge. Let it warm slightly before re-whipping if chilled.