Pour 2 tablespoons of the heavy cream into a small bowl. Sprinkle the gelatin in an even layer over the cream and set it aside to bloom.
In a large bowl, whisk together the egg yolks, sugar, and salt. Set the bowl aside.
In a saucepan over medium-high heat, combine the whole milk and vanilla. Heat the milk until bubbles form around the edges, whisking constantly to avoid scorching, about 5 minutes, then immediately remove the pan from the heat and reduce the heat to medium-low.
Whisking constantly, drizzle ½ cup of the hot milk into the bowl with the egg yolks, then return the mixture to the saucepan of milk.
Cook the custard mixture, constantly whisking, until it begins to boil and thicken, about 10 minutes.
Meanwhile, heat the bloomed gelatin in the microwave in 10 second increments until it's hot to the touch.
Remove the saucepan from the heat and stir the hot gelatin into the mixture until well combined.
Transfer the custard into a large bowl and immediately press a piece of plastic wrap onto the surface. Chill the custard for at least 2-3 hours, until completely cooled.
When ready to finish, whip the remaining heavy cream until medium peaks form.
Remove the custard from the refrigerator. Working in 3 batches, use a rubber spatula to gently fold the whipped cream into the custard, incorporating each batch fully before adding another, and trying not to deflate the whipped cream.
Divide the mousse into individual serving bowls, then refrigerate for at least 2 hours, or until the mousse is set. Serve with whipped cream, fresh fruit, or chocolate sauce
Notes
Gelatin powder helps stabilize the custard mixture. Give it at least 20 minutes to bloom before adding to the custard. I do not suggest using gelatin sheets.Vanilla bean paste adds a concentrated vanilla flavor and specs in the mousse. As a substitution, you can use an equal amount of good quality vanilla extract.Store covered in the refrigerator for up to 5 days.Freeze covered in an airtight container for up to 2 months. Defrost in the refrigerator before serving.