This is an easy and wonderful tasting black bean puree. Latin american cooking has three basic types of beans: whole cooked, puree'd, and re-fried. All are good, but this is probably my favorite.
Excellent as a side to any Caribbean or Latin themed main dish. 
Prepare the vegetables:
Put the pressure cooker pot on medium heat and add the oil. When the oil begins to shimmer, add the butter.
If using the bacon, add now and cook until the bacon starts to brown.
Chop the vegetables and add to the pan, cooking on medium heat until they are softened and well combined.