Wash the prawns, scallop and caul fat. Strain and pat dry with paper towel.
Divide the prawns into quarters or fifths along its length.
Divide the scallops into halves or quarters (depending on their size).
In a bowl, season the prawns and scallops with sugar, salt, chicken powder, potato starch and oil. Set aside to marinate.
Line a flat area with glad wrap and place a chopping board on top, closer to your seat.
Spread 1 layer of caul fat on the chopping board (if its area is bigger than the chopping board, let it rest flat on the glad wrap).
Scoop 2 tablespoons of the marinated seafood filling onto the caul fat in the shape of the seafood roll.
Use a sharp knife or a pair of scissors to cut the caul fat into a rectangle around the seafood filling. There should be about 4cm (1 1/2 inches) of caul fat around each side the filling.
Wrap the cut caul fat around the seafood filling as you would an egg roll (lift the bottom up to just cover over the filling, fold the left and right sides of the caul fat in to seal the sides, place your hands on the covered filling and roll it tightly in an upward direction until it forms a roll). Set aside.
When all seafood filling is wrapped, lightly coat with potato starch. Shake off excess starch.
Pour oil into a wok. Heat over medium-high heat and cook the seafood rolls in batches for 3-4 minutes or until golden. Drain on paper towel.
(Optional) Coat each seafood roll in egg white one at a time before you fry.
Serve seafood rolls with mayonnaise.