Add the softened butter to a large bowl and beat with an electric mixer (or silicone spatula) for 3–5 minutes, until pale, smooth, and fluffy. This step is important. The longer you beat the butter, the lighter your buttercream will be.
250 grams Unsalted butter
Sift in the icing sugar in two stages, mixing slowly in between with a silicone spatula to avoid a messy sugar cloud. Scrape down the bowl as you go.
450 grams Icing sugar
Add the milk (or cream), vanilla extract and salt if using, then beat with your electric mixer (or silicone spatula) until the buttercream is combined and smooth.
2 tablespoons Milk, 1 teaspoon Vanilla extract, Pinch Salt
Check the consistency: If the buttercream feels too stiff, add a splash of milk. If it’s too soft, add a tablespoon more icing sugar.
Remove the air bubbles: After all that mixing, it's normal for the buttercream to contain lots of little air pockets and bubbles. To remove them, use your silicone spatula to smooth and spread the buttercream around the side of the bowl in a circular motion. If you listen closely, you'll hear the little bubbles pop. After about 5 minutes, your buttercream should look completely smooth and have a light, creamy consistency.