Bloom the yeast by mixing the warm water, sugar, and yeast. Let it sit for 5 to 10 minutes until foamy.
In a large mixing bowl, combine the all-purpose flour, bread flour, white sugar, salt, evaporated milk, bloomed yeast, and shortening.
Mix until a dough forms.
Knead for 10 to 15 minutes until smooth and elastic. Perform the windowpane test.
Lightly coat the dough with cooking oil and place it in a bowl.
Cover and let the dough rise for about 1 to 2 hours or until doubled in size.
Divide the dough into 11 to 12 equal pieces, approximately 60 grams each.
Shape each portion into a smooth ball.
Arrange on a lined baking tray with enough space between each piece.
Cover and rest for 15 minutes.
Gently flatten each dough ball.
Cover again and let rest for another 45 minutes.
Prepare the custard by combining the egg yolks, evaporated milk, and condensed milk.
Reserve 2 tablespoons of the mixture for brushing later.
Add the flour to the remaining custard mixture.
Cook over medium-low heat while stirring continuously until slightly thick, or when it starts coating the back of a spoon. Use a balloon whisk if lumps appear.
Let the custard cool slightly.
Transfer the custard into a glass or small pitcher for easier pouring.
After the dough has rested, press the center of each bun using the bottom of a floured glass.
Fill the center with custard cream.
Brush the sides of the dough with the reserved custard mixture.
Bake in a preheated oven at 170°C for 18 to 20 minutes.
Allow to cool slightly before serving.
Enjoy your homemade custard cream buns!