Cut the cold butter into cubes size 3-4 cm and leave it slightly to soften the surface of the cubes should be soft, but the center should still hard. Mix the egg whites with the sugar and salt in a metal bowl. Put the pot on the stove (with a diameter less than a bowl of egg whites) with a little water to boil. Put the bowl of egg whites and sugar over it and start stirring constantly with a wire whisk until the mixture temperature reached 60C. If you do not have cooking thermometer, not scary. Dip your fingers from time to time in the mixture and once you feel that you begin to money, you've reached the required temperature. This whole procedure takes a few minutes about 5 minutes, so be careful to not cook them egg whites. When you dip your fingers should not feel sugar. Pour the hot mixture into the bowl of a stationary mixer or another bowl and start beating. Beat with a mixer (with wire attachment for mixing eggs) at high speed until you get a hard, shiny snow that does not fit when you tilt the bowl. Change the attachment of the mixer. Place one that is mixing of viscous mixtures (no batter) and looks like a shovel or rather pedal, if you so you can continue with the same. Add half the diced and slightly softened butter. Quickly on and off the mixer several times until the butter is completely covered with beaten whites. Add the remaining butter and repeat with turning on and off a few times. Turn the mixer on low speed and start beating. Starting from the lowest increase speed every 10 seconds until you reach the average high. Crush until you get a smooth and fluffy mass. At one point, the mixture may seem like a truncated, but this should not bother you. Just proceed with mixing and everything will be fine. Stop the mixer, knock down the walls of the bowl with a shovel and add rum (coloring, flavoring). If liquefy after adding the butter do not worry and put it for 20 minutes in the refrigerator and shake again. Cream can be stored for up to 2 weeks in the refrigerator or frozen before use was stirred at room temperature.