Preheat oven to 350.
Butter 2 cookie sheets and refrigerate for 15 minutes.
Cream butter and sugar until fluffy.
Beat in egg yolks and vanilla until incorporated.
Gradually add flour and salt and beat until well mixed.
Transfer to a pastry bag with a star tip.
Pipe onto sheet while nearly touching tip to pan. Leave at least 1" between cookies.
Decorate with sprinkles, other decorations or chopped nuts.
Bake for 11-14 minutes or until the edges are barely brown.
Transfer to a wire rack to cool.