In a medium saucepan, combine the first portion of thickened cream with the scraped seeds and pod of the fresh vanilla bean. Heat over low heat until it's steaming but not boiling, about 5-7 minutes. Stir occasionally to prevent sticking. Remove from heat, and carefully strain the mixture to remove the vanilla pod, leaving you with a fragrant infused cream.

In a separate mixing bowl, vigorously whisk together the large eggs, caster sugar, and cornstarch until well combined and slightly frothy, about 2-3 minutes.

Gradually pour the warm vanilla-infused cream into the egg mixture, whisking continuously to temper the eggs and prevent curdling, about 1-2 minutes.

Return the combined mixture to the saucepan and cook over low heat, stirring constantly until it thickens, aiming for between 180-183°F (82-84°C), about 5-6 minutes.

Transfer the thickened custard to a clean bowl and cover the surface with plastic wrap to prevent a skin from forming. Place in the refrigerator for at least 1-2 hours, or until fully cooled.

In a separate bowl, whip the remaining thickened cream until stiff peaks form, about 3-5 minutes. Be careful not to over-whip.

Gently fold the whipped cream into the chilled custard in 3-4 additions, using a spatula and a gentle motion.

Carefully spoon or pour the mousse mixture into serving cups, filling them about three-quarters full. Refrigerate for at least 3 hours, or overnight for best results.