Whisk the all-purpose flour, cornstarch, sugar, baking powder, and salt in a large bowl. In a separate bowl, whisk milk, egg, and 1 tablespoon of oil until mixed well. Pour the wet ingredients into the dry ingredients and whisk until you have a smooth, thick batter – it should be thick, but still pourable. Let the batter rest for 5 minutes.

Take out the skewered sausages from the refrigerator. Put the flour and panko breadcrumbs in two separate wide, shallow dishes. Roll each sausage in flour first, shaking off excess. Pour the batter into a tall glass or narrow container. Dip each floured sausage into the batter and let the excess batter drip off. Immediately roll the battered sausage in panko breadcrumbs and press. Place on a plate and repeat with the remaining sausages.

Heat oil in a deep pot or kadhai to 170-175°C – the oil should be deep enough to cover the corn dogs. Test with a small piece of bread; it should turn golden in about 30 seconds. Carefully lower 2-3 corn dogs into the hot oil, holding them by the sticks. Fry for 4-6 minutes, rotating occasionally, until they are a deep golden brown and crispy all over. Remove and drain on paper towels. Repeat with the remaining corn dogs.

While still hot, you can sprinkle the corn dogs with a little sugar for the traditional Korean sweet-savoury combination. Drizzle with ketchup, mustard, and mayonnaise in zigzag patterns. Serve immediately while hot, for the best cheese pull effect.