While the crumble bakes, make the custard sauce.
If using whole cardamom pods, use a mortar and pestle to smash the pods and get the seeds out. Then remove the pods and just grind down the seeds. If starting with just seeds of course, start by grinding those! A spice grinder will work too!
Separate the eggs and place the egg yolks in a medium heat safe mixing bowl. Save the egg whites in the fridge for something else!
Pour the milk into a medium sauce pot along with the ground cardamom and place it over medium low heat until it starts to steam.
While the milk heats up, whisk the sugar into the egg yolks along with the salt.
Place the mixing bowl on a damp rag to keep it from spinning.
Once the milk is steaming, turn off the heat and slowly pour the milk into the egg yolk mixture - whisking continuously.
Then pour the whole custard mixture back into the pot.
Heat over medium low heat, using a rubber spatula to stir the custard constantly. The mixture will thicken slightly and the small surface bubbles will disappear which is when you'll know it's ready to remove from the heat.
Turn off the heat and pour the custard through a fine mesh sieve into a medium heat safe bowl and whisk in the butter and vanilla. If you don't have vanilla bean paste, vanilla extract will work too!
Cover the surface of the custard with a sheet of plastic to prevent it from forming a skin. Leave it to cool to room temperature - if making ahead you can then place it in the fridge.