Preparation

Heat oven to 350F.  Mix together shortbread cookies, and butter.  Press firmly into the bottom of 4 greased 4 ounce ramekins.  Bake crust for 10 minutes and cool.  Crust will be very soft when hot but will harden as it cools.  Turn oven down to 300F.  In an electric mixer, combine the cream cheese, mascarpone, vanilla extract, and salt.  Mix until smooth, scraping down the sides as needed.  Add the eggs one at a time, completely incorporating before adding the next.  When the eggs have been added, pour batter into the ramekins to just below the top.  Bake for 18 to 22 minutes.  The cheese cake will rise slightly, be a light golden brown on top and still be slightly wobbly in the center.  Carefully remove from oven and cool on the counter for an hour then transfer to the refrigerator for an additional two to three hours or until completely cool.  While cheese is cooling, Melt chocolate in the microwave or a double boiler.  Combine the yolks, sugar and water in a metal bowl and whip over a double boiler until it thickens and becomes pale yellow in color.  It should be the consistency of soft whipped cream.  Add the vanilla and remove from heat.  Pour in the melted chocolate and fold together with a rubber spatula.  The egg mixture will collapse and thicken when the chocolate is added.  When the egg and chocolate are completely incorporated, fold in the whipped cream ¼ at a time.  Transfer the mousse to a container to chill.  Chill for 2 hours before frosting cheese cakes.  When cheese cakes are completely cool, scrape around the edges with a paring knife to loosen while holding upside down to remove from ramekin.  Use a piping bag or spatula to top the cheese cake with the cold chocolate mousse.