This is an ingenious way to make butter light and airy, an invention of my friend Hervé This, one of the founders of molecular gastronomy.
Makes a whole lot of wonderfully airy butter. Can be frozen.
Make sure the butter is room-temperate when you start cooking.
Arrange a cold-water bath. Use two large bowls, one bigger than the other. The smaller bowl (where you are going to make the beurre chantilly) should be small enough to fit inside the large bowl. It is advisable that the big bowl is quite stable, and that the smaller bowl has a rounded bottom, which simplifies the whisking process. Fill ice water into the large bowl, and set aside. Put the butter in the other bowl.
Heat the cognac in a small pot over medium high heat. When steam starts to rise, ignite with a match or a lighter. When the fire has burned out, add the cognac to the butter. Remove the bay leaves, and whisk until the butter has melted. Let it cool down a bit, before you transfer it to the cold water bath. You want the ice to stiffen the butter, not the butter to melt the ice.
Transfer the mixture to the cold water bath. Using a proper whisk (preferably a balloon whisk), whisk continuously until the butter is cold. Start whisking energetically. When patterns start to form on the surface, beat as much air as you can into the butter. Continue until it is thick and creamy and peaks are forming on the surface. Transfer to a serving bowl and keep refrigerated until half an hour before serving.