Whip on speed 3 until medium peaks form. Add powdered sugar on speed 1. Whip on speed 3 until stiff peaks form. Steadily add fat on speed 1, adding 1 to 2 lbs at a time. Mixture might break but will come together after all fat has been incorporated.
Scrape bowl well.
Whip on speed 2 or 3 for 2 to 3 minutes, until buttercream is smooth and thick. Do not overwhip or buttercream will become airy. Store in bucket in refrigerator. Shortening 6 lb. Egg whites 25.5 lb.