To prepare the filling, in a pan over medium-low heat, add the cooking oil and stir-fry the dried shrimp for about 1 minute until fragrant, then add the scallions and mix well.
To make the sauce, in a bowl, mix the hot water and sugar until dissolved, then add the soy sauce, dark soy sauce, oyster sauce, and sesame oil and stir to combine.
To prepare the batter and tray, in a bowl, whisk the wheat starch, tapioca starch, rice flour, water, and cooking oil until smooth, then brush a thin layer of cooking oil onto a heat-resistant tray, stirring the batter again before each use.
To steam the cheung fun, in a steamer over high heat, bring the water to a boil, pour a thin layer of batter into the tray to coat the bottom, place it into the steamer, cover, and steam for about 3 minutes until slightly translucent, then remove and let rest for 1 minute.
To fill and roll, on a lightly oiled surface, loosen the edges of the sheet, place it smooth side down, add the filling across the center, and gently roll it up.
To repeat and serve, continue with the remaining batter and filling, then serve warm with the prepared sauce drizzled over the top.