In a large bowl, sift together the flour, cornstarch and salt. Whisk the egg with the water, then whisk into the flour mixture until smooth.
In an 8-inch nonstick skillet, heat 1 tablespoon of the oil until shimmering. Add one quarter each of the scallions, red pepper and jalapeño and cook over high heat until barely softened, about 1 minute. Add one quarter each of the shrimp and squid, scattering them evenly in the pan. Pour in 1/3 cup of the batter, tilting the pan to spread it. Cook over high heat until the bottom is crisp and browned, about 3 minutes. Using a spatula, carefully flip the pancake and cook on the other side until set, about 20 seconds. Slide the pancake onto a plate and make 3 more pancakes with the remaining ingredients. Cut them into quarters, or serve whole, with Soy Dipping Sauce.