Cream the butter and sugar with a stand or hand-held mixer fitted with paddle attachment. Add egg yolk , salt and flour and mix until just combined. Roll dough between two sheets of parchment paper and place on a cookie sheet. Chill until firm.
Heat oven to 350F. Cut cookies out and place on parchment-paper lined cookie sheet. Bake until just golden on the edges.
Once cooled, dip tops of cookies into icing. Shake, flip and allow to dry.
Royal Icing
Beat all ingredients until smooth.Divide into bowls to color and add a touch of water, if needed, to thin out.