Cut nian gao and sweet potato into about 1/4-inch slices and about the same size. They won't be perfect square or rectangles, but that's okay as long as they are roughly about the same size and thickness
Prepare crispy batter:
Once you have that ready. Prepare the crispy batter by mixing the flour, salt, and baking powder and whisk to mix
Preheat about 1-inch of cooking oil for deep-frying. When you are ready to fry, pour the ice-cold water into the flour mixture and whisk to mix until combined
Once the oil is hot, lower the heat to medium, pick up the sandwich and coat it with the batter and fry until golden brown or the sweet potatoes are cooked through, about 3-4 minutes. Make sure the heat is not too high or the outside will brown too quickly before the inside cooked through
Remove and place on an absorbent paper towel, repeat with frying another 3-4 at a time. Place the fried nian gao on a cooling rack in the oven to keep them warm while you are frying the rest. The nian gao sandwich will stay crispy for roughly about one hour at room temperature
You can use 2 slices of sweet potatos to sandwich 1 slice of nian gao OR 2 slices of nian gao to sandwich 1 slice of sweet potato. It's really up to you and there's no right or wrong here.