Melt the butter in a medium saucepan over medium-low heat. Add the brown sugar and corn syrup. Cook, stirring constantly, until the sugar is dissolved, about 5 minutes. Remove from the heat, whisk in the salt, milk and vanilla, then the powdered sugar, whisking until smooth. Dip the filled doughnuts into the glaze, then set on a wire rack to set the glaze for 15-20 minutes.
After the glaze cools, it will thicken more, so you can gently rewarm it on the stove to thin it out again.
Yields enough glaze for about 2 dozen doughnuts.
Recipe from Curly Girl Kitchen