In a medium saucepan, combine Part 1 of the heavy whipping cream with the seeds scraped from a fresh vanilla bean pod and the pod itself. Heat over medium until it simmers gently, then remove from heat and let it infuse for about 15 minutes.

In a large mixing bowl, whisk caster sugar and large eggs until pale and slightly thickened. Sift in cornstarch and whisk until fully incorporated.

Strain the infused cream into the egg-sugar mixture while whisking. Remove the vanilla pod and transfer mixture back to the saucepan, cooking over low heat, stirring constantly for about 5 minutes until thickened.

Remove from heat and let cool to room temperature. Cover and refrigerate for 1 to 2 hours until chilled.

In a clean mixing bowl, whip the second half of the heavy whipping cream until stiff peaks form, about 3-5 minutes.

Gently fold the chilled custard into the whipped cream in three to four additions.

Spoon the mousse into serving cups, smooth the tops, cover, and refrigerate for at least 3 hours or overnight.