01 - Whisk together rice flour, tapioca starch, water, salt, and 1/2 teaspoon oil in a bowl until completely smooth. Let the batter rest for 10 minutes to allow proper hydration.
02 - Finely chop the peeled shrimp. Heat 1 tablespoon oil in a skillet over medium heat. Sauté the chopped shallot until fragrant, about 1 minute. Add the chopped shrimp, salt, sugar, and pepper. Stir-fry until shrimp are fully cooked and slightly dry, approximately 4-5 minutes. Remove from heat and set aside.
03 - Heat 2 tablespoons oil in a small pan until hot but not smoking. Pour the hot oil over sliced scallions in a heatproof bowl. Mix gently and set aside to infuse.
04 - Bring water in a steamer to a boil. Lightly grease small 3-inch ceramic dishes or ramekins. Stir the rested batter and pour approximately 2 tablespoons into each greased dish, creating a thin layer. Arrange dishes in the steamer, cover, and steam for 7-8 minutes until cakes are set and translucent. Remove from steamer and let cool slightly before unmolding.
05 - Combine fish sauce, sugar, lime juice, water, and sliced chili in a small bowl. Stir vigorously until sugar completely dissolves. Adjust balance to taste if needed.
06 - Top each steamed rice cake with a small spoonful of cooked shrimp mixture. Drizzle with scallion oil, sprinkle with crispy fried shallots, and add mung bean paste if using. Serve immediately while warm with dipping sauce on the side.