In a medium saucepan, heat the milk with the vanilla pod on medium-high heat until it is just at a simmer - don't let it boil.

In a large mixing bowl, whisk egg yolks, sugar, salt and cornstarch until glossy.

Place the bowl on a dampened kitchen towel to stop it from slipping around for this next step. 

Once the milk is hot, gradually pour it into the egg mixture, whisking constantly. This is tempering the eggs.

Return the combined mixture to the saucepan over medium heat. Cook the vanilla custard, whisking constantly until it starts to bubble.

Once it starts bubbling, it will begin to thicken, keep whisking to avoid the bottom of the pan burning. Keep it at a bubble don't stop whisking for around 1 minute. This is important to cook the custard properly so it stays thick once cooled.

Remove from the heat and stir through the butter.

Transfer the crème patissière to a bowl, cover with plastic wrap (directly on the surface to prevent a skin), and let it cool to room temperature.

Prepare the crème chantilly 

​When the pastry cream has cooled, begin the cream.

In a separate bowl, whip the cold heavy cream and powdered sugar to firm peaks. Take are not to overwhip it or it will become grainy.

Gently fold half the whipped cream into the cooled pastry cream. Follow with the remaining whipped cream, until there are no streaks of white cream remaining.

Cover and chill the crème légère until ready to use.