Whisk the all-purpose flour, sweet potato starch, and salt in a bowl. Slowly whisk in the water until smooth, then whisk in 1 tbsp of the neutral oil; cover and rest 15 minutes (thin, pourable batter). Reserve the remaining neutral oil for the pan.
Heat a 10-inch nonstick skillet over medium heat. Lightly brush with some reserved oil. Pour in about 0.5 cup batter and quickly swirl to a thin round. Cook until the surface turns matte and edges lift, 45–60 seconds; flip and cook until set with light spots, 15–30 seconds. Slide out and repeat to make 4 wrappers, stacking under a clean towel.
For each bing (4 total), heat the skillet over medium and brush with a little reserved oil. In a small bowl, beat 1 egg with about 2 tbsp sliced scallions and a pinch of white pepper, then pour into the skillet and spread gently. Immediately lay one wrapper on top, press lightly so the egg adheres, and cook until the egg is set but still tender, 45–60 seconds; flip and cook 15–30 seconds more.
Roll up the dan bing, or fold into thirds, and cut into bite-size pieces. Serve hot with Taiwanese sweet chili sauce or soy sauce for dipping.