Remove husk from tomatillo and rinse to remove any residual stickiness. Peel two of the four shallots and slice each in half. Toss the tomatillo, halved shallots, and jalapeno with one tablespoon olive oil. Place the vegetables on a baking sheet (cut-side up for the shallots) and roast for 15 minutes. Allow to cool.
Meanwhile, sauté the pancetta in a medium nonstick skillet or sauté pan over medium heat until crispy. Remove pancetta with a slotted spoon or spatula and set aside on a paper-towel-lined dish (but don't clean out the skillet). Add the other tablespoon of olive oil to the pancetta drippings. Peel and thinly slice the remaining two shallots and sauté in the pan until just beginning to caramelize.
When cool, slice the jalapeno lengthwise and remove the stem and seeds. Peel and scoop out the avocado flesh, adding it with the jalapeno, cilantro and roasted vegetables to a food processor. Blend until creamy. Taste, and add salt and pepper to your liking.
Heat a large skillet or sauté pan over medium heat. Butter each slice of bread and put butter-side down in the pan. Top with the pepper jack, guacamole, grilled shallots, and pancetta, and remaining bread slice. Note that, depending on how big your slices of bread are, there may be extra guacamole. . Grill until bread is golden brown, flip, and repeat. Serve with any remaining guacamole and chips on the side.