Combine 1/2 cup water, yeast, 2 teaspoons sugar, and 1 teaspoon cake flour in a large bowl; let stand until top is beginning to form a layer of white foam, about 20 minutes. Add remaining cake flour (2 cups minus the 1 teaspoon) and stir gently using chopsticks until dough comes together. Knead dough until smooth. Cover with plastic wrap and let first dough stand in a warm place for 4 to 6 hours.
Mix 2/3 cup water, ginger, spring onion, oyster sauce, 1 tablespoon sugar, all-purpose flour, 1 tablespoon corn flour, sesame oil, and white pepper in a saucepan over medium-low heat; cook, stirring constantly, until thickened, about 5 minutes. Cool filling to room temperature; refrigerate.
Spoon chilled filling over pork; mix well.
Sift 1/2 cup corn flour, 3/8 cup cake flour, and baking powder together into a bowl. Add 3/8 cup sugar and cooking oil and mix well. Mix corn flour mixture into first dough, kneading until smooth. Roll dough into a ball. Cover ball with an inverted bowl and let stand for 10 minutes.
Divide dough into 12 to 15 pieces and roll each into a ball. Flatten the balls into rounds. Spoon 1 to 2 tablespoons filling into the center of each round and fold round around filling and seal. It's fine if the top is a bit thick, it just helps create the top flower look.
Line bamboo steamers with parchment paper and arrange buns on top. Set aside in a warm place for 5 minutes. Close lids tightly.
Bring 1 1/2 to 2 quarts water to a boil in a wok or large skillet for 10 minutes. Place steamers in the wok and steam buns over medium heat until cooked through, 12 to 15 minutes.
