Cut a 3 by 5-inch template out of lightweight cardboard or stack several index cards together and wrap in plastic wrap leaving 1 side smooth and set aside. Line 2 baking sheets with parchment paper or nonstick baking mats; set aside. In the bowl of an electric mixer fitted with the paddle attachment, cream the butter and sugar for 1 minute. Add the vanilla, flour, and salt and mix until just combined, about 1 to 2 minutes more. On a lightly floured surface, roll dough out to 1/4-inch thick. Place the smooth side of the template onto the dough and cut out 16 rectangles using a paring knife; re-rolling dough as needed. Transfer rectangles to prepared baking sheets using a wide spatula. Chill for at least 1 hour. Preheat oven to 325 degrees F. Bake cookies until they are just beginning to color and are lightly firm to the touch, 15 to 18 minutes. Remove to cooling rack; let cool completely. Make the royal icing. Divide icing evenly between 4 bowls and cover each tightly with plastic wrap. Set 1 bowl aside for outlining the flag. Add red food coloring to one of the bowls and mix thoroughly, adding more food coloring, if necessary, to obtain desired color. Add 2 to 3 teaspoons of water to icing so that is loose enough to flow easily off the spatula when lifted and forms a ribbon that dissolves almost immediately. Replace plastic wrap to cover tightly. Repeat this process with 1 of the 2 remaining bowls of icing using blue food coloring. To the remaining bowl of icing add just the water and mix thoroughly. Recover all bowls tightly with plastic wrap. Transfer the reserved outlining icing to a small pastry bag fitted with a #5 tip. Pipe an even line on the outside edges of the cookie to outline. Pipe a vertical line 1 1/2 inches long 1/3 of the way from the left side of the cookie. Pipe a horizontal line from the left border to meet the bottom of the first line. This forms the outline of the field of stars. Pipe 3 horizontal lines evenly spaced from the original vertical line to the right side border, making sure the third line from the top is even with the corner of the field of stars. Pipe 3 more horizontal lines evenly spaced from left to right under the field of stars. Repeat this process on the remaining cookies. Reserve the icing in the pastry bag for the stars. Allow the icing to dry for at least 1 hour. Place the remaining 3 icings each into a squeeze bottle. Flood the field of stars with enough blue icing to fill the space evenly without overflowing. Use the tip of a small knife or a wooden skewer to push the icing completely into the corners. Using the same technique, flood every other stripe with the red icing. Let dry at least 2 hours. Flood the remaining 3 stripes with the red icing. Change the tip of the pastry bag with the reserved white icing to a #16 tip. Pipe as many stars onto the blue icing as will fit; let dry. Store in an airtight container at room temperature for up to 1 week.
In the bowl of an electric mixer fitted with the paddle attachment, combine confectioners' sugar and meringue powder or egg whites. Mixing on low speed, add a scant 1/2 cup water drop by drop. For a thinner consistency, usually used for flooding, add more water. A thicker consistency is generally used for further embellishing. Mix until icing holds a ribbon-like trail on the surface for 5 seconds when you raise the paddle. Yield: 2 1/3 cups
NOTES: For dark colors such as black and red, start light and work towards a deeper color. Add color with a toothpick, a tip full at a time, mixing thoroughly with each addition. Color will fully develop as it sets, so wait about 1 hour for the final result. Mix in daylight to better see the true color of icing. Always use clean toothpicks with each new addition of food color. Food color has a long shelf life. Any contamination with perishable ingredients, however, will hasten spoilage. Cover each bowl of icing with a damp towel to prevent it from hardening or forming a "skin". Work with clean piping tips to ensure application of the right color. Place a damp paper towel in the bottom of a juice glass to hold piping tips. Keeping tips damp when not in use will prevent icing from drying out and clogging the tip opening. Flooding: This is the process of coating the cookie with royal icing. Use a #1, #1.5, or #2 tip held 1/4-inch above the cookie. Follow the outline of the cookie, leaving space along the edge for spillage. Fill in outline with icing. To close empty spaces, push icing in place with a clean toothpick.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.
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