How to Make It
In the bowl of a standing electric mixer fitted with the paddle, mix the flour with the sugar and salt. Add the butter and mix at low speed until the butter is broken up into small pieces, about 2 minutes. Increase the speed to medium and mix until the flour and butter form small clumps, about 1 minute. Add the egg yolks and vanilla and mix at low speed until the dough comes together in a few large clumps. Pat the cookie dough into two 1/2-inch-thick disks, wrap them in plastic and refrigerate until chilled but not firm, about 30 minutes.
Preheat the oven to 375°. Line 2 large rimmed baking sheets with parchment paper. On a lightly floured surface, working with 1 disk at a time, roll out the dough 1/8 inch thick. Using 3- to 4-inch cookie cutters, cut the dough into shapes and transfer to the prepared baking sheets. Reroll the scraps and cut out more cookies. (Alternatively, cut the dough into 3- to 4-inch shapes and, using a smaller cutter, stamp out the center of half of the cookies.) Refrigerate the cutout cookies until chilled, about 30 minutes.
Bake the cookies for about 13 minutes, until they are lightly golden around the edges; shift the baking sheets from top to bottom and front to back halfway through for even baking. Let the sugar cookies cool on the baking sheets for about 5 minutes, then, using a metal spatula, carefully transfer them to a rack to cool completely, about 20 minutes.
For sandwich cookies, spread a thin layer of raspberry or apricot jam on the solid cookies and top with the corresponding cutout cookies to expose the jam. Otherwise, decorate as desired with the Royal Icing, sprinkles, sparkles and dragées.