Bake and cool cake. Trim off center doorway and turret area of cake so that surface will be level for starburst message.
In advance, use royal icing to make 120 (40 each yellow, red, blue) Tip 16 stars on board covered with waxed paper. Make extras to allow for breakage and let dry.
Ice sides and background area of cake smooth with chocolate buttercream icing. Outline center starburst message area with Tip 3, fill in area with Tip 9 (smooth with finger dipped in cornstarch). Outline rockets with Tip 3 and cover with Tip 16 stars.
Trim 5 lollipop sticks to staggered lengths; attach a yellow star to bottom of each with icing. Insert sticks into bottom of cake.
For each star spray, twist 5 florist wires together at bottom. Randomly attach stars to wires with royal icing. Insert flower spikes in cake and position a star spray in each. Use Tip 3 to outline burst marks and to print message on cake. Add Tip 21 star bottom border.
The 4th of July calls for a bit of shine and sparkle. This easily portable sheet cake is decorated with star-shaped cookies brightened with red and blue decorating gels. Light the candles and let the fireworks begin!