In the pressure cooker over medium heat, melt butter. Add leeks and potatoes and stir frequently for 2 minutes. Add the chicken stock, salt and pepper. Close the lid of the pressure cooker. Program 10 minutes into the timer and select program 2 - grains.
Turn the heat to high. After hearing the first audible signal, reduce the heat to low. At this point the countdowns begins. A gentle release of steam will be constant from the pot. Upon the second audible signal, turn off the heat source and move the program selector to pressure release.
Once the program is finished, open the pressure cooker.
With an immersion blender, or food mill or blender, mash all the vegetables to a rich paste like consistency.
Ladle into soup bowls and sprinkle with chives.
Optional: Off the heat and just before serving into soup bowls add the whipping cream and stir well. Or ladle on bowls, place a dollop of crème fraiche in the middle and sprinkle the chives over it.