Preheat oven to 350 degrees F.
To make the cake, cream the brown sugar with the shortening in an electric mixer. Add the buttermilk and vanilla then the melted chocolate. With the mixer running, add eggs, 1 at a time. Sift the flour with the baking soda, and salt. Add the dry ingredients to the sugar mixture and beat for 2 minutes.
Grease and flour a 13 by 9 by 2-inch rectangular cake pan. Pour the batter into the prepared cake pan and bake until the cake is springy and a toothpick can be inserted and removed cleanly, about 40 minutes. Allow the cake to cool for 10 minutes then turn it out onto a rack to finish cooling.
To make the sauce: Melt the 2 chocolates with the butter in a saucepan over very low heat. Stir in the powdered sugar, alternating with evaporated milk and blending well. Stirring constantly, bring the mixture to a simmer over medium heat. Simmer until the mixture becomes thick and creamy, about 8 minutes. Stir in the vanilla. Serve slices of cake topped with warm fudge sauce.