Beat first 2 ingredients and 6 Tbsp. water at low speed with an electric mixer until blended. Beat at high speed 4 minutes or until stiff peaks form. Add up to 2 Tbsp. additional water, 1/4 tsp. at a time, until desired consistency is reached.
Use less water for a stiff icing perfect for attaching cookies to cake stands. Use more water for thinner icing suitable for piping delicate designs on cookies.