In a bowl of a stand mixer fitted with a paddle attachment add the butter, salt, and icing sugar. Beat at medium speed until combined. The mixture should look like fine bread crumbs.
Add the cream cheese to the powdered sugar mixture. Beat at low speed until combined. Scrape down the sides of the bowl and beat again at high speed until light and fluffy, about 5 minutes.
Add the milk to the frosting, scrape down the sides of the bowl, and mix again.
Once the frosting is light and fluffy, add the Biscoff butter and vanilla. Beat at high speed until combined, scrape the sides of the bowl, and then mix again. Fill a piping bag with the Biscoff frosting and use it to decorate immediately.