By SadieBear, member
Posted: February 17th, 2013
Whisk the flour, baking powder, and salt in a medium bowl until combined.
In a stand mixer fitted with the paddle attachment (or in a large mixing bowl with a hand-held electric mixer), beat the butter and sugar on medium speed until light and fluffy, 3 to 4 minutes. Scrape down the bowl and beater with a rubber spatula as needed. Add the egg, zest, and vanilla and continue to mix until well blended, about 1 minute. Add the flour mixture and mix on low speed until well blended, about 1 minute more.
Turn the dough out onto a lightly floured work surface and gently knead until it's smooth with no air pockets. Using a large piece of plastic wrap as an aid, shape the dough into a 12-inch-long, 2-inch-wide log. Twist the ends of the plastic to compact and secure the log. To help the dough keep its shape, set it in an empty cardboard paper towel tube that's been slit lengthwise and freeze until very firm, at least 2 hours. Alternatively, give the log a quarter turn every 15 minutes during the first hour of freezing to keep it from flattening on one side. (At this point, the dough can be frozen for up to 1 month. Thaw in the refrigerator for 1 hour before slicing and baking.)
Position racks in the upper and lower thirds of the oven and heat the oven to 350 degrees F. Line 2 or more cookie sheets (preferably flat sheets with one raised edge) with parchment or nonstick baking liners.
Roll the log of dough in sanding sugar until evenly coated. Using a sharp, thin knife, slice the dough a scant 1/4 inch thick and set the cookies about 1 inch apart on the prepared sheets. If the dough becomes too soft to hold its shape when slicing, refreeze until firm; if it's too hard to slice, let it sit at room temperature until sliceable.
Bake two sheets at a time, rotating and switching their positions halfway through, until the edges and bottoms of the cookies are golden, 6 to 8 minutes. Let the cookies cool on their sheets on wire racks for about 5 minutes and then transfer them to the racks to cool completely.
Put the powdered egg whites and 6 Tbs. warm water in a stand mixer fitted with the whisk attachment (or in a large bowl if using a hand-held electric mixer). Let stand for 2 minutes, then whisk on low speed until combined. Let stand 2 more minutes so the powder can absorb the water, and then mix on low speed until completely dissolved, about 1 minute more. Increase the mixer speed to medium and beat until frothy, about 1 minute. Add the confectioners' sugar and mix on low speed until blended, about 1 minute. Increase the speed to high and beat until the mixture is thick and shiny, 3 to 5 minutes. The icing should be able to hold a ribbon, but if it doesn't, add more sugar, a little bit at a time.
Add the lemon zest and stir until combined. To make several colors, divide the icing into small batches in separate containers. Use a toothpick or cake tester to add a small amount of food coloring to the icing, stirring until no streaks remain, and adding more color as needed.
If not using the icing immediately, put a damp paper towel directly on the surface to prevent a skin from forming; you can let the icing sit at room temperature for up to 2 hours. If not using within 2 hours, cover the bowl with plastic wrap and refrigerate for up to 12 hours. Let the icing come back to room temperature before using. If the icing stiffens, add a few drops of water and beat briefly on low speed to loosen.
To outline a cookie: Spoon some of the icing into a pastry bag fitted with a very small plain tip. Practice first on a piece of cardboard or waxed paper; if the icing is too thick to pipe evenly, put it back in the bowl and stir in water, a drop or 2 at a time, until it pipes easily but still retains its shape. Outline the cookie with the icing. At this point, set the outlined cookies aside to dry until tacky, about an hour (depending on the humidity of your kitchen), or decorate the outline further while it's still wet with dragees and sanding sugar.
To coat an entire cookie with icing (flooding): Thin some of the icing by stirring in cold water, 1 tsp. at a time, until just pourable; you don't want flooding icing to hold its shape but rather fill in and 'flood' an area on the cookie, so the consistency should be similar to that of melted chocolate. Spoon the flooding icing into a pastry bag fitted with a medium plain tip. Pipe a tight zigzag of flooding icing inside the border of an outlined cookie and then use a toothpick to spread the icing in an even layer out to the edges.
Decorate the cookie further with dragees and sanding sugar, and then set the cookie aside to dry completely, 2-1/2 to 4 hours.
Serve or store decorated cookies in an airtight container at room temperature for up to 3 days.