Melt butter in a medium saucepan over medium heat.

Once butter has melted, stir in powdered sugar, salt and milk. Whisk until sugar has dissolved completely.

Stir in baking soda. Whisk for about 30 seconds until frothy.

Remove from heat. Stir in vanilla.

Serve immediately or store, in a sealed container, in refrigerator up to one week. Reheat before serving.