Sift the flour, potato starch, baking powder and salt into a medium bowl. In a small bowl, whisk the egg white until frothy. Whisk the stock into the flour mixture, then whisk in the egg white. Whisk until smooth. Let the batter stand for at least 1 hour.
Pour 1 1/2 tablespoons of vegetable oil into an 8-inch cast-iron skillet and set over moderately high heat for 1 minute. Arrange one-sixth of the scallion greens on the bottom of the skillet in a neat layer, all facing in the same direction; cook for 30 seconds. Pour 1/3 cup of the batter over the scallions. Scatter one-sixth of the shrimp and one-sixth of the squid over the batter and top with one-sixth of the red pepper strips. Stir the egg yolk with a fork, then decoratively drizzle a little of it over the pancake and cook until the underside is golden, about 4 minutes.
Using 2 spatulas if necessary, carefully flip the pancake; drizzle a little oil under it if the skillet looks dry. Cook the pancake for 2 to 3 minutes longer, until golden brown on the bottom. Turn the pancake out onto a plate and repeat to make 5 more pancakes. Cut the pancakes into wedges with scissors and serve hot, with the Soy Dipping Sauce.