Preheat oven to 400° F. Line baking sheets with parchment paper and set aside.

In a bowl, whisk together the flour, baking powder, salt. Set aside.

In a large bowl, combine softened butter and confectioners’ sugar using a hand mixer. Beat until light and fluffy, about 1 to 2 minutes. Add the egg and vanilla, and beat until thoroughly incorporated, about 30 seconds. Scrape down sides as necessary.

With mixer on low, slowly add flour mixture to the bowl. The dough is ready when most of it sticks to the paddle. When touched, it has a little give, but does not stick to fingers. If it's too sticky, add more flour, 1 tablespoon at a time.

Flour the work surface and roll dough to about 1/4-inch thick, using flour for dusting as necessary. Use a 2 ½ to 3 inch star-shaped cookie cutter to cut cookies and place on prepared baking sheet. Gather scraps and re-roll dough when necessary. If baking in a warmer climate, chill cookies for 20 minutes before baking.

Bake for 7 to 8 minutes, rotating halfway through. Allow cookies to cool on baking sheet for 2 minutes, then transfer to wire rack to cool completely. Prepare icing.

Place the meringue powder and 5 tablespoons water in a large mixing bowl, and whip with the whisk attachment until foamy, about 3 minutes.

Add the vanilla and confectioners' sugar, then whip on medium speed until thick, about 3 minutes. The royal icing should hold a medium soft peak. If necessary, thin with additional water, a few drops at a time, to get the desired consistency.

Transfer half the icing to a large icing bag with a round tip. Cover the bowl with the remaining icing with a wet paper towel so the icing doesn’t dry out. Outline the cookies, then flood them with white icing. Use a toothpick if needed to spread the icing into any bare spots. Let the cookies dry completely (this could take a few hours).

Divide the remaining icing into two bowls. Add red food gel to one bowl and blue food gel to the second bowl. Transfer to two separate icing bags, each fitted with a small round tip (I used Wilton #2).

After the icing on the cookies is dry, use the red and blue remaining icing to add firework designs on top. Dry completely.