In a mixing bowl, combine the evaporated milk, water, and instant yeast. Stir gently until the yeast is dissolved.

Add the bread flour, cornstarch, salt, sugar, egg, and softened butter. Mix until a cohesive dough forms.

Transfer the dough to a lightly floured surface and knead for 8–10 minutes, or until smooth and elastic.

Place the dough in a lightly greased bowl, cover, and allow it to proof in a warm environment for approximately 1 hour, or until nearly doubled in volume.

In a separate mixing bowl, combine the all-purpose flour, cornstarch, baking powder, butter, sugar, and egg. Mix until a smooth dough forms.

Divide the cookie dough into 10 equal portions. Cover and refrigerate until ready to use.

Once the bread dough has proofed, gently punch it down and divide it into 10 equal portions. Shape each portion into a smooth ball.

Place the topping sugar in a shallow plate. Roll each portion of cookie dough into a thin round, press the top side into the sugar, and place it over a bread dough ball. Gently cup and smooth the cookie dough over the surface.

Using a sharp knife, lightly score a grid pattern on the cookie dough surface.

Arrange the buns on a lined baking tray, allowing sufficient space between each. Cover and allow to proof for 1 to 1½ hours, or until noticeably puffy.

Preheat the oven to 350°F (175°C).

Bake the buns for 15–20 minutes, or until lightly golden and fully baked.

Transfer to a wire rack and allow to cool completely before serving.