Make the Annatto Oil: In a small saucepan, combine the oil and annatto seeds and bring to a simmer over low heat. Remove the pan from the heat, cover, and let the oil cool completely, about 30 minutes. Strain the Annatto Oil into a jar.

Make the chowder: In a large, shallow glass or stainless steel bowl, toss the shrimp with two-thirds of the minced garlic, the scallions, lime juice, and 1 teaspoon of salt. Cover with plastic wrap and refrigerate for at least 1 hour or for up to 3 hours.

In a food processor, puree the corn with the milk. Pour the puree through a coarse strainer, pressing on the solids to extract as much liquid as possible.

Heat the Annatto Oil in a large saucepan or enameled cast-iron casserole. Add the remaining garlic, onion, bell pepper, and cumin and cook over moderate heat, stirring, until the vegetables are slightly softened, about 5 minutes. Stir in the tomatoes and cook for 2 minutes longer. Add the corn milk, stock, plantain, cilantro, and cayenne and bring to a boil. Simmer over moderately low heat until very flavorful, about 20 minutes.

Pour the soup through a coarse strainer. Working in batches, puree the vegetables in a blender. Return the puree and the strained broth to the saucepan and bring to a simmer. Add the shrimp and its marinade and cook over moderate heat until the shrimp are just opaque throughout, about 2 minutes. Season with salt and serve in warmed soup plates or bowls with the Tangy Corn Salsa.