Cut banana leaf into 18cm x 15cm rectangles. Scald the leaves to soften.
Shell the prawns, leaving tails intact. Make a slit on the back of each prawn and de-vein. Press out flat and open up butterfly style. Set the prawns aside. Blend the spice ingredients in a food processor into a fine paste.
Combine coconut milk with rice flour, corn flour and seasoning ingredients. Add beaten eggs and kaffir lime leaves. Stir well to mix. Add prawns and scallops to the spiced custard mixture.
Take a piece of banana leaf, form a box-like shape by folding the sides and secure with bamboo toothpicks or staples. Line the base with two kadok leaves. Put a prawn and a scallop on the kadok leaves and spoon over 2-3 tablespoons of custard mixture; cover with another kadok leaf. Steam the bundles of otak for 12-15 minutes over rapidly boiling water or until custard sets.