Melt the butter in the pressure cooker pot over medium-low heat. Add the leeks, celery leaves and 1 teaspoon of the salt and cook, stirring often, until soft, approximately 8 minutes. If the leeks start to brown during this process, reduce the heat.
Drain the potato pieces and add to the pot along with the simmering chicken broth. Clamp on the cooker's lid, (according to the manufacturer's instructions) and bring to full pressure over high heat.
Once pressure is attained (there will be a loud whistle) back the heat down to medium-low, just maintaining a hissing of steam for 8 minutes.
Kill the heat and release the steam pressure by opening the dump valve or setting the cooker under cold, running water. When the lid can be removed, carefully transfer the mixture to a blender carafe. (If the mixture fills your carafe more than two-thirds full, work in two batches!)
Top the carafe with the main lid but remove the center lid so that air can escape as it heats and expands. Cover the lid tightly with a tea towel and blend on low for 1 minute before slowly increasing the speed to smoothly puree the potatoes. (Although you want to increase the speed slowly, try not to puree the potatoes too long as they could become gummy.)
Transfer the mixture back to the pot and whisk in the cream. Stir in the white pepper. Taste and adjust the seasoning (I always need the rest of the salt). Chill thoroughly then serve garnished with a small spoonful of the roe and dill fronds.
Tightly covered, the vichyssoise will keep a week in the refrigerator. The soup can also be frozen in ice-cube trays then kept in freezer bags for up to three months.
Strictly speaking, you don't have to do this in a pressure cooker. You can use a regular pot but it will take quite a bit longer and you should start with about 1/4 cup more broth to make up for evaporation during the process.
This recipe has been updated and may differ from what was originally published or broadcast.
Recipe courtesy of Alton Brown and Elizabeth Ingram