Prepare the Dough : In a mixing bowl, combine the flour, instant dry yeast, sugar, and salt. In a separate bowl, mix the warm water, milk, and oil. Gradually pour the liquid into the dry ingredients, mixing with a spoon or your hands until a soft dough forms. Knead the dough on a lightly floured surface for about 8–10 minutes until it becomes smooth and elastic. Cover the dough with a damp cloth and let it rise in a warm place for about 1 hour, or until it doubles in size.

Make the Custard Filling: Gently heat the milk in a small saucepan over medium heat until it’s warm, but not simmering or boiling. In a separate bowl, whisk the sugar, egg yolk, and cornstarch until smooth. Slowly pour the warm milk into the egg mixture, whisking constantly to prevent curdling. Return the mixture to the saucepan and cook over low heat, stirring continuously, until thickened. Gently heat the milk in a small saucepan over medium heat until it’s warm, but not simmering or boiling.

Assemble the Buns: Once the dough has risen, divide it into equal portions (about 8–10 pieces). Place a spoonful of custard filling in the center of each flattened dough piece. Carefully seal the edges, forming a smooth ball, and pinch to seal tightly. Place the buns seam-side down on a tray or plate and let them rest for 15–20 minutes.

Cook the Buns : Heat a non-stick skillet or pan over low heat. Cover with a lid and cook for 8–10 minutes on one side. Flip the buns and cook for another 5–7 minutes until both sides are golden brown and cooked through. Remove from the pan and let cool slightly before serving.