Combine powdered sugar, shortening, salt. Mix in milk of choice – perhaps about 1/8 cup – until reaching the right consistency.
Mix and mix until nice and creamy. Add in vanilla after the frosting is mixed; otherwise it will bind with the powdered sugar and you will have lumps.
It needs to be a little stiffer for the gingerbread house than if you were decorating a cake.
Using more shortening makes it creamier. Using less shortening and more milk will make it harden better and be less creamy and sweeter.