Put the jalapenos under the broiler for about 5 minutes per side, then let cool, split them lengthwise and scrape out the seeds, then dice.
Saute the bacon in your soup pot until crispy, stirring over medium heat. Remove the bacon with a slotted spoon onto a paper towel. Pour off all but 1 Tablespoon of the grease.
Saute the celery and onion over low heat until soft, stirring periodically (8-10 minutes). Stir in the chopped garlic and cook a minute, then add 4 cups of broth, potatoes, cumin and cayenne. Turn the heat up and bring to a gentle simmer. Cook about 30 minutes, uncovered, until the potatoes are tender, stirring now and then.
If you're using an immersion blender, blend the soup in the pot. If you're using a blender or food processor, let the soup cool ten minutes or so before blending and do it in batches.
Return the soup to low heat and stir in the bacon, jalapenos, cilantro, buttermilk and half and half. Add more broth, water, buttermilk or half and half if you want a thinner soup. Heat the soup over low heat for about 10 minutes, then add salt and pepper to taste.
Serve with sour cream, grated cheese and green onions.