To make the Sweet Thai Chilli sauce, bring the water, fish sauce and sugar to a boil in a saucepan over medium heat, and simmer for 2 to 3 minutes, stirring constantly, until the mixture turns into a thin syrup. Remove from the heat and set aside. About 10 minutes before serving, combine all the other ingredients with the syrup and mix well. Transfer to a serving bowl.
Grind the peeled prawns or shrimp to a paste in a food processor. Combine the prawns or shrimp, pork (if using), salt, fish sauce, sugar pepper in a large bowl and mix until well blended.
Wet your hands, spoon 1 tablespoon of the mixture and shape if into a ball, them press if flat to from a round cake. Coat the cake on all sides with the breadcrumbs. Continue until all the mixture is used up.
Heat the oil in a wok or saucepan until hot. Gradually lower the coated cakes into the oil, a few at a time, and deep-fry for about 5 minutes, turning occasionally, until golden brown on all sides. Remove and drain on paper towels.
Line a serving platter with the pineapple slice (if using) and arrange the deep-fried cakes on top. Serve hot with a bowl of Sweet Thai Chilli Sauce or Chinese plum sauce (if using) on the side.