Whisk the flour, baking powder, and salt in a medium bowl.
Beat the butter and both sugars in another medium bowl with an electric mixer on medium-high speed until light and fluffy, about 30 seconds. Add the egg yolks, vanilla and orange zest mixing until fully incorporated. Slowly add the flour mixture, and continue beating until the dough comes together, stopping and scraping down the sides of the bowl as needed.
For rolled cookies: Roll about a tablespoon of dough by hand into a ball. Dip 1 side of the balls into some coarse sugar and place them sugar-side-up on an ungreased baking sheet, leaving about 1-inch between cookies.
For sliced cookies: Divide dough in half, roll by hand into 2-inch-wide logs, wrap in plastic wrap and refrigerate until firm, at least 2 hours.
Cut the logs into 1/4-inch-thick cookies and place them on ungreased baking sheets, leaving about 1-inch between cookies.
For cutout cookies: Divide dough in half, pat into disks, wrap in plastic wrap and refrigerate until firm, at least 2 hours.
Roll dough between lightly floured parchment, or waxed paper, until about 1/3-inch thick. Transfer sheets to a baking sheet and refrigerate until firm, about 30 minutes. Cut into desired shape using a cookie cutter, place them on ungreased baking sheets, leaving about 1-inch between cookies. (Gather the dough scraps together, pat into a disk, chill and reroll.)
Refrigerate cookies while preheating the oven to 375 degrees F, for at least 30 minutes.
Bake the cookies, until the bottoms are golden, about 10 to 15 minutes depending on shape. Cool on sheets until firm enough to transfer to a rack to cool. Decorate as desired and serve, or store in an airtight container at room temperature for up to 1 month.
Combine all the ingredients, except the food coloring, in the bowl of a standing mixer fitted with the paddle attachment. Mix slowly until stiff enough to form peaks. The icing should be pure white and thick, but not fluffy and bubbly. If the frosting is overbeaten, it will get aerated which makes it harder to work with. If this happens, let the frosting sit to settle, then use a rubber spatula to vigorously beat and smooth out the frosting.
Alternatively, combine ingredients in a large bowl, and beat with hand beaters on low speed until the frosting thickens to stiff peaks.
Add up to 1 tablespoon food coloring and mix with a rubber spatula until the color is uniform. (Adding too much color reduces the sheen of the frosting and can break down the consistency of the frosting over a couple of days.) Store icing, covered, with plastic film on the surface of the icing.
Yield: 1 pound royal icing (1 2/3 cup)
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