Trim the root ends off the scallions and slice thinly on the bias; you should have between 3 and 4 cups of sliced scallions. , In a large bowl, whisk together the flour, cornstarch, sugar, baking powder, and salt. Add the cold sparkling water and whisk to combine. Using a flexible spatula, fold in the scallions — it will look like a lot of scallions for the amount of batter.  
, In a medium nonstick skillet, heat 1 1/2 tablespoons of the oil over medium heat until hot. Ladle about 1/2 cup of batter into the pan (a scone and muffin scoop works well here) and use a spatula or the back of a spoon to evenly spread the batter into a thin circle between 6" and 8" in diameter. Cook the pajeon until the bottom is golden brown and crispy, about 3 to 4 minutes.  , Flip the pajeon and cook the other side until crispy, another 3 to 4 minutes. If necessary, turn the heat up towards the end of the cooking time to finish crisping the bottom. Transfer the pancake to a paper towel-lined cooling rack or plate, and cook the remaining batter, adding more oil between batches.  , While the pajeon cook, make the dipping sauce: In a small bowl, combine the soy sauce, rice vinegar, sesame oil, sugar, and gochugaru. Serve with the pancakes.  , Pajeon are best served immediately after cooking. Store in an airtight container in the refrigerator for up to 2 days; reheat in an oven or toaster oven on a wire rack set on a baking sheet.