In a bowl place the egg yolks, the whole eggs, the sugar, vanilla and mix very well with a mixer.
In a separate large bowl, mix the vanilla custard powder and the cornstarch with a pinch of salt. Mix until well incorporated.
Add the liquid mix over the dry ones mixing well until all incorporates well.
Add the milk and give it all a good whizz.
Butter an ovenproof dish and put it aside.
Put the mix into a saucepan and place over medium heat. Whisk continuously until it starts bubbling and until the custard begins to thicken. Put the heat on low and continue to mix until the custard is thick that you can see whisking trails in it. Switch off the heat.
Place the custard into the ovenproof dish, scatter the prunes around if you choose to put any in and put in the oven.
Bake for about 1 h 20' or slightly more until it is nice and has formed a nice brown skin.
Remove from the oven and let it cool.
Once completely cooled you can place it in the fridge for an hour or longer to make it ready to be served.