In the bowl of your stand mixer or using a hand mixer, beat the butter until light and fluffy, 3-5 minutes.
Sift in the powdered sugar, salt and vanilla and beat until well-blended. This is your buttercream base.
Over a double boiler, melt your white chocolate. Once fully melted, allow it to cool in a bowl. You want to keep your eye on it - check it after 5 minutes. Dip your finger in to test the temperature, once it’s cool to touch - you can move to the next step. Definitely don’t if it is still warm as it will melt the butter in your base.
Stir it into your buttercream base with a rubber spatula or with the mixer speed on low.
Let it rest for 10 minutes at room temperature, and then use immediately to frost cake.