Mix all but vanilla in 2 1/2 qt pot. Heat over med/low heat to a rolling boil. Boil exactly 1 min without stirring. Place pot with hot icing in a skillet full if ice to begin quick cool down. Beat icing in pot until smooth and spreadable (may be a few minutes). You'll be able to see large bubbles appear and the icing will begin to loose its shine as it thickens. Add vanilla and ice cake quickly as icing sets up fast.
It's important to use low/med heat so icing doesn't scorch. The skillet with ice cubes is the step that makes this icing soo creamy. Be sure to leave the icing in the pot and set the pot down on the ice cubes as you stir. Enjoy!