1. Using electric beaters or a stand mixer fitted with the paddle attachment, beat the butter until smooth. Add the icing sugar. Starting on low speed, work in the sugar, and then increase the speed to medium-high and beat until fluffy, about 2 minutes.
  2. Beat in the egg yolks and vanilla. Add the flour and salt and beat just until the dough comes together. Shape the dough into 2 discs, wrap in plastic and chill for at least 2 hours.
  3. Preheat the oven to 325°F (160°C) and line 2 baking trays with parchment paper.
  4. On a lightly floured surface, knead 1 disc of dough a little to soften it and then roll it out to just under 1/4 inch (6 mm) thick.
  5. Use a 2- or 3-inch (5 or 7.5 cm) cookie cutter to cut out cookies. Arrange on the baking trays, 1 inch (25 mm) apart. Repeat with the second disc of dough, re-rolling any scraps as needed.
  6. Bake the cookies for 12 to 14 minutes until slightly golden at the edges. Transfer to a wire rack to cool.
  7. Prepare the royal icing for flooding or piping. Decorate the cookies as you wish, sprinkling with decorator’s or sanding sugar or adding dragées or other edible décor. Allow 4 to 6 hours for the icing to dry.

Make Ahead: You can store these baked cookies, iced or plain, for 10 days in an airtight container at room temperature. They actually will keep for longer (about 1 month), if they are completely covered with royal icing. You can freeze undecorated baked cookies for up to 3 months. (Do not freeze decorated cookies as the icing may crack.) Thaw the cookies to room temperature before decorating.