Turn on the Instant Pot’s saute function to Normal / Medium.
Add butter and leeks. Saute until leeks are very soft, 8 to 10 minutes.
Add salt, black pepper, and bay leaves and saute for 2 minutes more.
Turn off the saute function.
To the bowl of the Instant Pot, add stock and potatoes. Scrape up any browned bits on the bottom of the pan (this is important because browned bits on the bottom of the pan may cause the burn error).
Close and lock the lid.
Set the pressure valve to Sealing.
Cook on manual / high pressure for 8 minutes.
Manually release any remaining pressure.
Remove and discard bay leaves.
Stir heavy cream into soup.
Use an immersion blender (or transfer to a standing blender) to blend soup until smooth. (Note: For the best texture, blend the soup as little as possible. Stop blending as soon as all of the potatoes are smooth. The thickness of the soup may vary slightly. Add some water a bit at a time, if needed, if the soup is too thick.)
Taste and add more salt and black pepper, if needed.
Serve with sour cream, crumbled cooked bacon, and / or chopped green onions on top.