Making the Vanilla Custard:

1. Separate the Eggs: Separate the eggs carefully, placing the yolks in a small bowl. Set aside the egg whites for another use or discard them.
2. Whisk the Yolks: In a separate bowl, whisk together the egg yolks, sugar, and cornstarch until the mixture is smooth and lump-free. If the mixture is too thick, add a tablespoon or two of milk to thin it out.
3. Prepare the Vanilla Milk: Split the vanilla bean lengthwise with a sharp knife and scrape out the seeds. Add the seeds and the pod to a small pot and pour the milk over it.
4. Heat the Milk: Heat the milk and vanilla bean over medium heat until it’s steaming but not boiling.
5. Temper the Yolks: Slowly pour a ladleful of the hot milk into the egg yolk mixture, whisking constantly. This process is called tempering and prevents the eggs from scrambling when added to the hot milk.
6. Cook the Custard: Pour the tempered egg mixture back into the pot with the remaining milk. Cook over medium-low heat, stirring constantly, until the custard thickens and coats the back of a spoon.
7. Strain the Custard: Pour the custard through a fine-mesh sieve into a bowl to remove any cooked egg bits or vanilla bean pod.
8. Chill the Custard: Cover the custard with plastic wrap, pressing it directly onto the surface to prevent a skin from forming. Refrigerate for at least 2-4 hours, or until completely chilled.

Making the Stabilized Whipped Cream:

1. Bloom the Gelatin: In a small bowl, sprinkle the gelatin over the cold water and let it sit for 5-10 minutes to soften.
2. Whip the Cream: Pour the cold whipping cream into a clean, chilled mixing bowl. Use a handheld electric mixer or a stand mixer with the whisk attachment to whip the cream to soft peaks.
3. Melt the Gelatin: Gently melt the bloomed gelatin in the microwave for about 20 seconds, or over a low heat on the stovetop. Do not overheat, as this will destroy its setting properties.
4. Incorporate the Gelatin: With the mixer running on low speed, slowly drizzle the melted gelatin into the whipped cream. Continue to whip until stiff peaks form.

Mixing the Mousse:

1. Loosen the Custard: Scoop about ¼ of the whipped cream into the chilled custard and gently stir to loosen it.
2. Fold in the Whipped Cream: Pour the custard mixture into the remaining whipped cream. Use a rubber spatula to gently fold the two mixtures together until they are completely combined. Be careful not to overmix, as this will deflate the mousse.

Serving the Mousse:

1. Scoop and Chill: Scoop the mousse into individual serving cups or glasses. Refrigerate for at least 2 hours, or until set.
2. Garnish: Just before serving, top the mousse with fresh berries, finely chopped fruit, a dollop of whipped cream, shaved chocolate, or a sprig of fresh mint.