Holiday Present and Star Shortbread Cookies

Prepare the Cookies: Preheat oven to 350°F. Line 2 baking sheets with parchment paper; set aside. Beat butter, powdered sugar, and vanilla with a stand mixer fitted with a paddle attachment on medium speed until smooth, about 1 minute. Add egg yolk, and beat just until combined, about 10 seconds, stopping to scrape down sides of bowl as needed.

Whisk together flour and salt in a medium bowl until well combined. Gradually add flour mixture to butter mixture, beating on low speed until combined and dough comes together, about 3 minutes.

Place dough on a lightly floured work surface; roll into a ¼-inch-thick rectangle. Cut dough into 1- to 1½-inch squares and rectangles and ¾-inch stars. Arrange cutouts 1 inch apart on prepared baking sheets. Gather and reroll dough once.

Bake in 2 batches on middle rack in preheated oven until edges start to darken slightly, 8 to 10 minutes. Cool on baking sheets 5 minutes. Transfer to a wire rack; cool completely, about 20 minutes.

Prepare the Royal Icing: Beat powdered sugar, 7 tablespoons warm water, and meringue powder with a stand mixer fitted with a whisk attachment on medium speed until fully combined and smooth, 1 minute, stopping to scrape down sides of bowl as needed. Increase mixer speed to high; beat until icing is no longer glossy, about 1 minute. Transfer ½ cup to a bowl, and stir in gold food coloring to tint it light gold. Repeat with an additional ½ cup icing and gold food coloring in a separate bowl to make a darker shade of gold. Leave remaining icing white. Cover bowls with plastic wrap; set aside at room temperature until ready to use.

Spoon icing colors into separate piping bags fitted with small round tips (such as Wilton #2 or #3), or cut a small corner from bottom of each bag. Pipe an outline of icing onto all Cookies, and fill in outlines with icing. Immediately sprinkle stars with gold or white sanding sugar. Let squares and rectangles stand until surfaces are set, about 30 minutes. Pipe remaining icing as desired to decorate squares and rectangles to look like presents. Let all Cookies stand until completely set, about 2 hours.