In a medium bowl, whisk together the egg yolks, sugar, cornstarch and salt aggressively for 1-2 minutes, until the mixture becomes smooth and runny, with no more chunks.
3 large egg yolks, 1/4 cup (45g) granulated sugar, 4 Tbsp (40g) cornstarch, tiny pinch fine sea salt
In a medium saucepan (preferably stainless steel or nonstick - avoid aluminum), heat the milk over medium heat, just until it comes to a simmer.
1 cups (8oz) whole milk
Temper the eggs by streaming half of the milk into the egg mixture while continuing to whisk.
Dump the whole mixture back into the saucepan and cook over medium-low heat, while whisking continuously.
Within 1-3 minutes, the mixture should start to thicken, continue whisking.
Once it's starting to stick to the pan, whisk aggressively for another 30-60 seconds. Remove from the heat and pour into a bowl with the butter and vanilla.
2 tsp vanilla bean paste, 3 Tbsp (42g) salted butter
Allow the heat from the pastry cream to melt the butter for a second and then gently whisk it. It may separate at first but it'll come back together as you whisk it.
The pastry cream should be smooth and silky now, if you have any lumps, feel free to push it though a sieve.
Cover the surface with plastic wrap and allow it to cool completely to room temperature. If you refrigerate it for a little, gently whisk it to get rid of any lumps before adding it to the whipped cream.
Place the whipped cream in the bowl of a stand mixer with the sifted powdered sugar, fit with the whisk attachment (or a large bowl with a hand mixer). Turn the mixer up to medium-low for a minute or so, until it starts to thicken (so it doesn't splash).
3/4 cup (6oz) heavy whipping cream, 1/4 cup (50g) powdered sugar
Then increase the speed to medium-high and beat just until stiff peaks form. Don't over-mix.
To bloom the gelatin, mix it with warm water until it's dissolved. Microwave for 3-5 seconds if you need to.
1 tsp gelatin powder
Add the dissolved gelatin, chilled pastry cream and any flavors you want to the whipped cream and mix on medium speed, just until it's all combined. Use a rubber spatula to scrape the bottom of the bowl to make sure it's all mixed well.
Use immediately as a filling for cakes, cream puffs, eclairs, etc.