Clean your chicken well, under cold running water. Focus on the cut surfaces and if you see blood coming out, squeeze the muscle tissue around the area to coax out any blood remaining in the meat. If you are using whole carcasses, use a spoon to scrape out all the dark organ bits inside the carcass, especially along the spine and ribs. Your chicken should be as clean and blood-free on the inside as it is on the outside.
In a 7-10 quart pressure cooker, add the chicken and water and bring it to a rolling boil. Use a skimmer to scrape away the foam that accumulates on the surface. Keep skimming until there's no more foam coming to the surface.
When the stock is clean, add your aromatics and turn the heat off.
Secure the lid to the pressure cooker and then turn the heat back on high. Let the cooker come up to pressure (it will start whistling) and then turn the heat progressively down until you hear a slow gentle whistle. If the whistling stops after a while, turn the heat back up slightly. Set a timer for 1 hour.
After the chicken cooks for an hour you can either let the pressure drop naturally or opt for one of the quick release methods for your particular cooker.
After the pressure drops and you open up the cooker, use a large whisk to whisk chicken stock, breaking up the chunks into a pulp. This does two things. This releases the collagen in the meat and skin into the soup, and is what gives your soup its thick luscious texture and creamy taste.
After you're done whisking, pass the stock through a colander into another pot or large bowl, then press on the solids with the back of a ladle to squeeze out as much flavor as you can from the pulp.
Return the solids to a pot and add enough water to cover the solids. Bring the mixture to a boil, and then whisk and strain this into the bowl with the other stock, pressing on the solids to get as much stock out as you can.
Discard the solids, and then wash out the pot. Use a fine mesh (double mesh) sieve to strain your stock back into the pot and let this cool to room temperature.
While your stock is cooling, it will separate into layers, and you'll see the oil float to the top. This is part of what makes the stock so creamy, so you need to emulsify this back into the mixture. The best way to do this is by using an immersion blender, but if you don't have one, you can just pour the stock into a regular blender in batches and blend it until the mixture is milky and white. Divy this up into containers and store it in the fridge.
The stock will solidify in the refrigerator, this is the collagen (a.k.a. gelatin) at work and means you made a good stock. To use the stock, just reheat it and whisk to emulsify the mixture.