In a large bowl, vigorously whisk egg yolks, sugar, and cornstarch until the mixture is pale yellow.
In a small saucepan, heat your milk, vanilla, and salt over medium-low heat until it just comes to a simmer (do not boil). Remove from heat.
Whisking your egg yolks constantly, slowly stream the milk into egg yolks to temper the eggs. Once the mixture is fully incorporated, return everything to the saucepan.
Place the pan over medium heat, and whisk constantly until the mixture is thick like a pudding, and has come to a boil (you will see thick bubbles rise from the bottom and pop).
Remove from heat and strain over a fine mesh sieve into a heat-proof bowl.
Whisk in your black sesame paste until smooth. Fine, yes, go ahead and taste it - I couldn't help myself either.
Press a sheet of plastic wrap directly onto the surface of the cream and refrigerate until cold. This will take up to about 4 hours. In the meantime, you can proceed to make the choux pastry.
Once cold, loosen with a whisk.
In a small, microwave-safe bowl, let the cold water and gelatin powder sit for 5 minutes until the gelatin blooms. Microwave 20 seconds (just until the gelatin melts) and stir until smooth - set aside to cool while you continue.
Begin whipping your heavy cream until slightly thick. Still whipping, beat in your gelatin. Whip at high speed until you reach stiff peaks.
Fold in your black sesame cream to your stabilized whipped cream.
Transfer to a large piping bag and refrigerate till ready to use.