Whisk the egg yolk in a small bowl with a bit of milk, vanilla powder, and cornstarch until you have a smooth mixture.
Pour the rest of the milk into the saucepan and heat it up over medium heat.
Add the powdered sugar to the milk, and whisk until it has dissolved.
Scoop two heaping tablespoons of pistachio paste into a strainer and push the paste through the strainer with the back of the spoon while holding it over the pan. This ensures you filter out the bigger chunks of pistachio and leaves you with a smoother pudding in the end.
Whisk the pistachio paste and milk together until there are no lumps of the paste left in the mixture. In later steps, the pistachio paste will fully mix in with the milk.
Remove the saucepan from the heat and let the mixture cool down a little bit. Add tiny increments of hot milk to the egg mixture while whisking. This ensures the egg heats up slowly and stops it from curdling.
Add the egg mixture to the pan and heat everything on low to medium heat. You'll see that the mixture will thicken up quickly, so keep whisking until you are left with a thick, smooth pudding.
Let the pistachio pudding cool down for at least an hour in the fridge.
Finish the pudding off with a tuft of whipped cream just before serving. Enjoy!