Put all ingredients including bread flour, all-purpose flour, sugar,salt, baking powder, dry yeast, butter, beaten egg, and warm milk together. Knead the ingredients either by hand or machine for 20 minutes until they become smooth, well mixed dough.
Place the dough into a bowl and cover the top, then set it aside until it has at least doubled in size. How long it takes for that to happen depends on the room temperature. It normally takes about an hour for me.
While we wait for the dough to ride, we can make the custard cream. Mix 3 egg yolks and sugar. Whisk until the color turns creamy yellow. Then, add cornstarch and whisk more. Then add 50ml of warm milk and whisk again.
Pour the egg mixture into a pot of remaining warm milk (200ml). Add vanilla extract. Constantly whisk it over low-mid heat until the texture becomes thicken.
Transfer the custard cream into a bowl and cover with a plastic wrap. Make sure to press it against the custard cream. So that it won’t develop a slimy film on the surface. Immediately place the cream in the fridge to cool.
Next, let’s make a whipped cream. Whisk whipping cream and sugar together until almost over-beaten. Place it in the fridge for later use.
Cut a parchment sheet into 12 small even size pieces.
After the 1st rise is finished, flatten the dough with floured hands and divide them into equal 12 sizes, 50g each. Place each dough on the small parchment sheet and set aside for 2nd rise. Wait until the dough is doubled in size. It takes 60-90 minutes depending on the room temperature.
In the meantime, let’s prepare the cinnamon sugar which we will be using later to coat the donuts. Mix sugar and cinnamon together in a small bowl.
Heat the cooking oil to 350F/180C over low-mid heat. Deep fry the donuts 1 minute each side. When the sizzling sound lessens, you can take the donuts out.
While they are very hot, dip them into the cinnamon sugar.
While waiting for the donuts to cool off, let’s prepare the whipped custard pastry cream. Combine the custard cream and whipped cream we made earlier. Transfer it into a piping bag.
Poke a hole with a knife. The depth of the hole is almost to the other end. Fill the hole with the pastry cream. Do the same for 12 donuts.