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1
For the shrimp broth, take the reserved shells (and heads), place into a pot, and add the water, a good pinch of salt, and the tomato paste (odd, I know, but I love the little bit of sweetness it gives, plus a hint of rosy color). Bring to a boil and then simmer, uncovered, until it reduces a bit, tasting and adjusting the salt as needed. When the stock is ready, strain it through cheesecloth to get rid of the shells and any foam or scum that has formed, then set aside or cool and store for future use. You should have about 4 cups worth.
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2
In a heavy-bottomed pot, melt the butter and add the diced pancetta, frying until it is crispy and golden, stirring occasionally. Remove the browned pancetta and drain it on a paper towel-lined plate. Add the leek, fennel, and celery, season with a pinch of salt, and stir to coat with the fat. Cook about 5 minutes until softened, then add the potato and the shrimp broth. Bring just to a boil, then cover, reduce heat to low, and cook just until the potatoes are tender.
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3
Stir in the half and half, season with pepper, taste and adjust the salt if necessary, and once the soup is back up to temperature, turn off the heat. Stir in the shrimp, cover, and let sit until the shrimp have cooked through in the hot liquid (this should take just a few minutes). Ladle the soup into warmed bowls and top with the crispy pancetta and snipped chives.
