Preheat the oven to 350 degrees F. Grease a 17 by 12-inch baking pan with 1 teaspoon of the shortening. In a large, heavy saucepan over medium heat, melt the remaining 1 cup shortening. Remove from the heat and let cool slightly. Add the sugar and blend. Add the eggs 1 at a time, whisking after each addition. Add the water and vanilla and whisk to mix well. Add the flour, cocoa, baking powder, and salt and stir to blend. Fold in the walnuts and chopped chocolate, then spread the batter evenly in the prepared pan. Bake until the cake springs back when touched, about 25 minutes. Remove from the oven and let cool for 30 minutes. Spread the icing over the cake with a rubber spatula. Let cool completely before cutting into squares to serve. Slice and serve individual pieces on a large platter.
Sift together the confectioners' sugar and cocoa powder into a large mixing bowl. Add the butter and use an electric mixer to combine. Add the water and mix until smooth. Let cool, then use.
Recipe adapted from Everyday is a Party, by Emeril Lagasse, with Marcelle Bienvenu and Felicia Willett, published by William Morrow, 1999