Mince the garlic and chile pepper. Sprinkle with salt and mash with the flat side of a chef's knife to make a paste. Heat 3 tablespoons olive oil in a large pot over medium heat. Add the garlic-chile paste and half of the scallions and fry until the paste is golden, 3 to 4 minutes. Add the shrimp shells and corn cobs and cook, stirring, until the shells are red, about 5 minutes. Add the wine and cook until almost evaporated, about 2 minutes. Add 6 cups water and bring to a boil over medium-high heat, then reduce the heat to medium low and simmer 15 minutes. Strain the broth and return to the pot; discard the solids.
Meanwhile, heat a large skillet over medium heat. Add the remaining 3 tablespoons olive oil, the remaining scallions and the corn kernels; cook until the corn is golden, about 12 minutes. Add 1/2 cup water and stir, scraping the pan with a wooden spoon. Transfer the corn mixture to a blender; add 1 1/2 cups water and 2 teaspoons salt and puree until smooth. Add the pureed mixture to the pot with the broth.
Add the squash, potato and evaporated milk to the pot. Bring to a boil over medium-high heat, then reduce the heat to medium low and simmer until the vegetables are tender, 12 to 15 minutes.
Add the mussels to the pot, cover and cook until they begin to open, about 4 minutes. Add the shrimp and flounder and simmer, stirring gently, until just cooked through, 3 to 4 minutes. Gently stir in the egg yolks and cook 1 minute, then remove from the heat, cover and let stand 3 minutes. Season with salt and top with the cilantro.