Boil the water with the butter in a small saucepan. Remove the pot from the heat and mix the flour with the cornstarch and pour into the hot liquid all at once. Mix everything together until smooth, then heat ("burn off") for about a minute, stirring constantly.

Put the dough in a mixing bowl. Work 2 eggs one after the other under the dough with a mixer (dough hook) at the highest setting. 
Stir the baking powder into the cold batter.

Using a piping bag with a large star tip, place 9 heaps of dough on the baking sheet and bake in a preheated oven at 200°C for about 20 minutes. Do not open the oven door during the baking time, otherwise the pastry will collapse.

Immediately after baking, cut off a top from each cream puff and let the pastry cool on a wire rack.

For the filling:

Cut a vanilla bean down the middle and scrape the seeds out. 
In a medium saucepan, heat the milk and vanilla bean by bringing it to a medium heat. Do not let it boil and immediately turn off the heat when bubbles form on the walls of the pot and set it aside to infuse for 15 minutes. 

Separate the egg whites and yolks into a separate bowls. Transfer the yolks into a large bowl along with the sugar. 

Whisk the yolks and sugar together until the color lightens and texture thickens.  Sift in the cornstarch and whisk vigorously.  

If you want to make both chocolate and vanilla cream separate the filling into two bowl and add cocoa powder to one bowl.

Whisk in 1/4 cup of the hot milk mixture until it is incorporated then slowly drizzle in the remaining hot milk mixture while whisking.

Place pot over medium-high heat and cook the filling while whisking continuously until thickened and slowly boiling. Now go ahead and remove from the heat and stir in the butter. 

Repeat the same with the chocolate cream and put the chopped chocolate bits with the butter at the end.

Let the custard cool slightly then pour it into a bowl. Cover with plastic wrap, lightly pressing the plastic against the surface to prevent a skin from forming. Chill it for at least 2 hours or until ready to serve.

Fill the creams into piping bags and fill the choux puffs with it.

I added one with only whipped cream and some strawberries to mimic the three versions Mashle has in its ending song!