Melt the butter and let it cool.

Meanwhile, boil the potatoes until tender and blend them into a purée using an immersion blender/blender.

Next, add the milk and egg whites to the mixture and strain it through a fine-mesh sieve into the whipped cream dispenser. 

If it is not to be served immediately, place it in a sous vide/water bath at 60 degrees Celsius (140 degrees Fahrenheit) until serving. 

Then, give it 3 cartridges and you'll get a fine foam for your dish.