Grease an 8 by 8 pan and then line it with parchment paper, with excess on the sides so that you can pull it out later. Beat the egg whites with the cream of tartar to soft peaks while the sugar, corn syrup, and water cook on medium-high heat in a medium saucepan. Cook the sugar until it reaches the soft crack stage, 270°F. Pour about a teaspoon of the sugar syrup into the egg whites and beat until incorporated. Add in another teaspoon and beat some more. Continue doing this until all the sugar syrup is incorporated. Add in the chocolate and the cocoa powder, and beat until the bowl is slightly warmer than room temperature, about three minutes. Pour into the pan. Spread the nougat into an even layer. It might be helpful to place a piece of parchment paper on top and press down on it with your hands to do this. Let the nougat sit for ten minutes at room temperature while you prepare the caramel.
Prepare the caramel:
Cook all the ingredients together in a medium saucepan over medium-high heat until the temperature reaches 245°F, about eight minutes. Do not stir. Pour onto the nougat, and let sit for two hours.
Cut the bars:
Take out the layers by pulling on the parchment paper and then place them on a chopping board, caramel side down. Cut the bars into any size you like. Place the layers in the freezer before cutting if you're having trouble. This will be sticky.
Dip the bars:
Dip the bars using a fork in the tempered chocolate, and then let the chocolate harden at room temperature. Store the chocolate bars in the fridge, and let them sit at room temperature for twenty minutes before serving.
Recipe by IronWhisk at http://www.ironwhisk.com/2012/12/homemade-mars-bars/