In a heat-proof bowl, pour boiling water over the corn husks. Lay a plate on top to keep the husks submerged. Let them rehydrate for a couple of hours until pliable. If you're doing this step ahead, store in a resealable bag in the refrigerator until ready to use.
Over an open flame or 4 inches below a preheated broiler, roast the poblanos, turning them regularly until evenly blistered and blackened. Place in a bowl, cover with a kitchen towel. Once handleable, peel off the skin, split in half lengthwise and remove the seed pod. Set 8 poblanos halves aside. Roughly chop the remaining poblano and scoop into a blender jar.
Melt the butter in a large (4-quart) saucepan over medium heat. Once hot, add the onions and cook for 5 to 6 minutes until golden brown. Scoop in the garlic and cook for 30 seconds or until it releases its aroma, then stir in the flour and continue stirring for 1 1/2 minutes to cook the flour. Raise the heat to high and whisking constantly, slowly add the milk and the broth. Bring to a boil, then reduce the heat to medium and simmer for 5 minutes. Remove from the heat and let cool slightly. Pour the sauce into the blender and blend until smooth. Taste and season with salt, usually 1/2 teaspoon. Let cool to room temperature before proceeding.
Preheat the oven to 425 degrees.
In a large bowl, combine the shrimp, corn, chayote, tomatoes, the cheese and the cooled poblano sauce. Transfer each of the prepared poblano halves to the center of a corn husk. Scoop a portion of the filling onto each poblano half. Overlap the ends of the corn husk to seal the poblano and filling. If your corn husks aren't large enough, use a second one. Twist the ends closed and, using one of the extra corn husks, tear off strips to use as a tie and secure each end of the corn husk. Transfer to a rimmed baking sheet and place on the center rack. Bake for 15 to 18 minutes or until the filling is hot.
While the shrimp rellenos are baking, prepare the panko crumb topping by mixing the panko, oil and paprika together. Scrape into a small skillet set over medium heat and toast until golden brown.
Remove the shrimp packages from the oven. Transfer them to individual plates with the poblano side facing up. Using a small paring knive, split the packages open by cutting down the center. Sprinkle the tops with the panko mixture and serve immediately.
Testing Note: Always shred your own cheese since the pre-shredded versions contain stabilizers that affect how well the cheese melts.