In a large bowl whisk together the dry ingredients: Rice flour, Mung bean flour, Tapioca starch, Granulated sugar and Fine salt until evenly combined.
In a separate bowl beat one Large egg with Water, Food-grade limewater (nam puen sai) and a little Neutral oil until smooth; gradually whisk the wet mixture into the dry ingredients to make a thin, pourable batter. Let the batter rest for 20–30 minutes to hydrate.
While the batter rests, make the sweet meringue: in a clean bowl beat the Large egg whites with additional granulated sugar and a pinch of Cream of tartar until glossy, stiff peaks form—this will be the airy topping that contrasts with the crisp shell.
Heat a nonstick skillet or small griddle over medium-high heat and lightly brush or spray with neutral oil.
Working quickly, pour or ladle a tablespoon or so of batter into the hot pan and tilt or swirl to form a very thin round (aim for a lacy, paper-thin edge). Cook 30–60 seconds until the edges lift and the underside is golden and crisp.
Immediately spoon or pipe a dollop of the meringue onto the center of the hot pancake and let it set for a few seconds in the pan—the meringue will hold its shape and warm through without fully melting.
Top the meringue with a small nest of Foi thong while still warm, then fold the pancake over like a taco or leave open-faced. Transfer to a wire rack so the shell stays crisp.
Repeat with the remaining batter and meringue. Serve the Khanom Buang warm or at room temperature for best texture—crispy shells with soft, sweet centers.
