Season chicken with salt and pepper. Roll in flour to coat.
Mix 1 cup water, sugar, lemon zest, and juice in a saucepan. Set aside.
Mix sweet rice flour with enough cold water to make a runny, pancake-like batter. Add a pinch of salt and pepper.
Heat oil in a wok to 400 degrees F (200 degrees C). Dip chicken into the batter. Deep-fry, a few pieces at a time, until light golden brown, 4 to 5 minutes per side. Drain oil and keep chicken warm on a serving platter.
Place lemon mixture over medium-low heat. Dissolve cornstarch in 6 tablespoons water and pour into the saucepan. Stir until mixture reaches a syrupy thickness. Add egg yolk; stir quickly until completely combined. Remove from heat.
Cut chicken into bite-sized pieces and serve with the lemon sauce. Garnish with lemon slices.