To make Choux:

Preheat oven to 395F (200C). Place parchment paper on a baking pan. Sift cake and all purpose flours together.

In a pot, heat water, oil, and salt together at medium heat. When it starts boiling, add the flour mixture and stir well with a spatula. Let the dough cook through until it becomes a ball.

Put the hot dough in a stand mixer and mix using a paddle to cool the dough down a bit. Beat eggs in a separate bowl.

Slowly add eggs in parts to the dough, forming a batter. Do not add all once. When you pull the paddle from the batter, the batter should be dropping and making a triangle shape. Adjust the amount of eggs depending on firmness of the batter.

Put the batter in a piping bag with a round piping tip. Pipe 2" (4.5-5cm) rounds on the parchment paper, 3" apart. Using a wooden skewer, on top of each round, make 4 lines from the edge to the center.

Bake in the preheated oven for 13 minutes. Turn down to 350F (180C), and bake further for 14-17 minutes.

To make Custard Cream:

Heat milk in a pot until just before boiling.

In a medium bowl, mix cornstarch and sugar with a whisk. Add eggs and mix very well.

Start adding hot milk to the egg mixture slowly. When all the milk has been mixed in, put the mixture back in the pot, bring to a boil, and cook for a couple of more minutes.

Remove from the heat and add butter and vanilla. Stir well to mix.

Let cool to room temperature and refrigerate at least a couple of hours.

To assemble:

Put the cream in a piping bag with a tip. Make small slits on the bottom of the Choux, and squeeze the cream into the shells. Dust with powder sugar if you like.