Place half of the powdered sugar and the meringue powder in the bowl of an electric mixer. Whisk together well.
Turn on mixer (use whip attachment) and, while motor is running, slowly stream in the water. Mix until everything is incorporated. Turn mixer to high speed and whip until stiff peaks form. Add flavorings and mix well.
Change to paddle attachment (for stand mixer) or dough hook (for Bosch). If using a hand mixer, use the same beaters you were using before. Add remaining powdered sugar and shortening and whip for 2-3 minutes more.