In a large bowl, beat the cream cheese and brown sugar together until smooth. Add the peanut butter and salt and mix well. Add the heavy cream, ¼ cup at a time, mixing very well in between each addition. Scrape the sides of the bowl with a spatula. Add the vanilla and mix very well until light and fluffy. It should be a mousse-like consistency. Pour and spread the filling over the crust and chill in the freezer for about a ½ hour or until the filling is set. Slice and serve. These bars are not the sort of bars you can grab and eat with your hands, so be ready with plates and spoons/forks. Keep these in the fridge at all times except for serving and eating. Enjoy!