Prepare/cook pie crust first.
Mix milk, butter, and sugar together in a small saucepan; set on the stove and bring to a boil.
Mix the wet cocoa powder and egg yolk together. Stir in a little of the mixture from the stove to slowly bring the temperature up. When hot enough, add the cocoa and egg into the saucepan and stir to make sure egg does not cook up in chunks.
Mix flour and cornstarch with a little cold water. Stir into the mixture and let boil a few minutes.
Pour mixture into pie crust and let cool. Refrigerate to set.
Beat the egg white and a little sugar to create a meringue. Spread over the pie and broil or use a kitchen torch to brown.