To the milk add about 1 tbsp spoon of milk and bring to simmer.
Add the rest of the sugar to the egg yolks and beat together until the mixture thickens and the color brightens, almost white.
Add the cornstarch and mix again to incorporate it.
Once the milk starts simmering, remove it from the heat and start adding it gradually to the egg yolks, sugar, and cornstarch mixture while continuously mixing for the eggs not to cook and curdle.
You don't have to add the entire simmered milk. Once the mixture is thin and the temperature is higher, add it to the rest of the milk on the saucepan, and bring it again to the stove.
While continuously mixing with a whisk, cook the cream on a medium to low heat until it thickens.
Remove from the heat and add the butter immediately and mix to incorporate it.
Divide the cream into 2 equal parts while it's still hot in order for the chocolate to melt.
To the first half, add chopped chocolate or chocolate chips and mix it until it's completely melted and incorporated. Cover with some cling wrap in contact with the cream and allow it to cool.
Similarly, add vanilla extract or vanilla paste to the rest of the cream and incorporate it. Cover to contact with some cling wrap and allow it to cool.
Before filling the choux, I suggest layering your dish or cups with plain choux to know how many you would need. Do not forget to double that number for more layers.
Once completely cooled, fill about a fourth quantity of each cream into piping bags and fill half the choux.
Fill the choux with the cream using a small nozzle, half with the chocolate cream and the other half with vanilla.