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.