Mix the flour and salt together.
Add the warm water, warm milk, oil, sugar, and instant yeast. Knead to form a dough.
Brush the dough with the egg white.
Optionally, add 1 tablespoon of vinegar to the dough for a lighter color.
Add 2 tablespoons of butter or oil to the dough.
Add 1 ½ tablespoons of cocoa powder to the dough.
Preheat the oven to 180°C (350°F).
Bake the pie for approximately 25 minutes, checking halfway through. If the top is browning too quickly, cover with parchment paper or aluminum foil.
Bake for an additional 10 minutes until fully cooked.
For the topping, mix the egg yolk, 2 tablespoons of milk, vanilla, and vinegar.
Brush the hot pie with melted butter or oil, then cover and let sit for 10 minutes.
Spread the topping mixture over the pie.
