Cream butter in mixing bowl. Gradually add sugar, beating until light and fluffy. Beat in egg yolk. Add cream and vanilla, mixing well. Stir in flour. Mix until blended. Spoon mixture into a pastry bag with a large open star, 2 cm diameter tip, filling bag half full (or use cookie press). Onto ungreased baking sheets, pipe out cookies, 5 cm apart. Sprinkle with nuts. Bake at 180ºC for 15 minutes. Remove cookies to racks for complete cooling.