Prepare Water Dough: In a mixing bowl, combine all-purpose flour and chilled, cubed unsalted butter. Use your fingertips to rub the butter into the flour until it resembles coarse sand. Add sugar, salt, the egg, and a splash of ice water, kneading gently until the dough is smooth and elastic. Wrap it in plastic wrap and let it rest for 15 minutes at room temperature.

Prepare Oil Dough: In another bowl, mix all-purpose flour with shortening and softened unsalted butter until you create a smooth, cohesive paste. Cover it with plastic wrap and let it rest for 10 minutes.

Assemble Doughs: Take the rested water dough and shape it into a cross on a lightly floured surface. Place the oil dough in the center and fold the ends of the water dough around it to encase it completely. Roll the dough out gently to flatten it, fold it into thirds like a letter, then roll it out again. Allow it to rest for 15-30 minutes.

Make Filling: In a bowl, combine hot water and sugar, stirring until dissolved. In a separate bowl, gently beat the eggs and then fold in the evaporated milk with a pinch of salt. Strain this mixture through a fine sieve to ensure a smooth custard.

Shape Tarts: Preheat your oven to 325°F (162°C). Roll out the prepared dough into thin circles, about ¼ inch thick, and place each into tart molds. Fill each shell with the custard mixture.

Bake: Bake the Hong Kong egg tarts for approximately 25-35 minutes, or until the custard is set and the crust is golden brown. Let them cool slightly on a wire rack before serving.