In a large bowl, whisk together the instant dry yeast, sugar, and salt before adding the all-purpose flour. In a separate container, mix the warm water, milk, and oil together. Gradually combine the wet mixture with the dry ingredients, stirring until a shaggy dough forms. Knead the dough on a floured surface for about 5 minutes until it becomes smooth and elastic.

Transfer the kneaded dough into a lightly oiled bowl, covering it with a clean kitchen towel or plastic wrap. Place the bowl in a warm, draft-free spot and let it rise for approximately 1 hour, or until it has doubled in size.

While the dough is rising, prepare the custard filling according to package instructions for the custard powder you've chosen. Stir until smooth and thickened, then set aside to cool completely.

Once the dough has risen, punch it down to release air, then knead it briefly on a floured surface. Divide the dough into 12 equal pieces, rolling each into a circle about 4–5 inches in diameter. Carefully place a generous spoonful of the cooled custard in the center, fold the dough over the filling, and pinch the edges to seal securely. Let the shaped buns rest for 15 minutes.

Heat a non-stick pan over low heat and lightly grease it if necessary. Place the rested buns in the pan, cooking them for about 10 minutes on one side, or until they are golden brown. Gently flip the buns and cook for an additional 5 minutes, ensuring they are cooked through and nicely browned.