Make the Shortbread Cookies

1. Assemble the dough. Add powered sugar, softened butter and vanilla to the bowl of a stand mixer fitted with the paddle attachment. Cream them together until light and fluffy – about 2 minutes. With the mixer on low speed, slowly add the flour and salt to the butter mixture. Mix just until the shortbread dough comes together. Be sure to not overmix.
2. Divide the dough in half. Shape each half into a disc. Tightly wrap the discs in plastic wrap and refrigerate for 30 minutes, or until the dough is chilled and firm. Line 2 cookie sheets with parchment paper. Put them aside.
3. Roll out the dough, and cut out the shapes. On a lightly-floured surface, roll out 1 disc of chilled dough to 1/4-inch thick. With a star and a rectangle cookie cutter, cut out as many 4th of July cookies as you can. Transfer the cut-out shapes to the prepared trays. Leave at least 2 inches between the cookies.  Gather the scraps of dough together. Re-roll and cut out more cookies until all the dough is used. Repeat with second disc of dough.
4. Preheat oven to 325°F. Chill the cut-out cookies. Place the baking sheets with the cut-out cookies in the fridge or freezer (either one will work) to chill for 30 minutes before baking. 
5. Bake the chilled cookies for approximately 12 to 14 minutes, or until the edges are slightly golden blonde. Remove the cookies from the oven. Let them cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely.

Decorate the 4th of July Cookies

1. Reserve white icing. When the cookies are ready to decorate, put 1 cup of the white royal icing into a piping bag fitted with a Wilton #3 tip. Divide the remaining icing between 2 bowls. 
2. Tint remaining icing in blue and red. Place 5 drops of blue food coloring into one of the bowls of icing. Stir to combine, adding more food coloring if needed to achieve your desired color. Transfer the blue royal icing to a piping bag also fitted with a Wilton #3 tip. Add 5 drops red food coloring to the last of the icing and stir to combine. Again, add more food coloring if needed and then transfer the icing to a piping bag fitted with a Wilton #3 tip.
3. Decorate the stars. Pipe an outline of blue icing around the edge of the cookie. Then fill in the center. Gently tap or shake the cookie to flatten and evenly distribute the icing. Allow the blue icing to set (about 10 minutes), and then sprinkle edible star glitter over the icing. Or you can pipe small stars onto the cookie using the white icing.
4. Decorate the flag cookies with stripes. Pipe 3 rows of red icing onto the cookies and allow them to set completely (about 10 minutes). Fill in the space between the red rows with white icing – a total of 2 rows of white icing. Then allow it to set.
5. Let the icing harden completely (for 2 hours) before storing. Place any leftover cookies in an airtight container and store them in the fridge for up to 4 days.