In a medium saucepan, combine granulated sugar and water over medium heat, stirring until the sugar dissolves. Allow this mixture to boil gently for about 4-5 minutes until it reaches a deep amber color. Remove from heat and stir in unsalted butter and salt until smooth. Gradually whisk in 1 cup of heavy cream, then set aside to cool.

In a separate bowl, whisk the large egg yolks until well mixed and slightly pale. Slowly pour the warm caramel mixture into the yolks while whisking continuously to temper them. Return the mixture to the saucepan and cook over medium heat, stirring constantly until it thickens, about 3-5 minutes.

Remove the saucepan from heat and stir in the vanilla extract. Allow the mixture to cool to room temperature. Transfer it to the refrigerator and chill for at least 1 hour.

In a large mixing bowl, beat 1 cup of heavy cream with powdered sugar and a pinch of salt until soft peaks form, about 3-5 minutes on medium speed.

Once the caramel custard has cooled completely, gently fold the whipped cream into the custard using a spatula.

Spoon the mousse into serving dishes and refrigerate for a minimum of 2 hours until fully set. Serve topped with whipped cream or a sprinkle of flaky sea salt.