For this recipe, it's best to use the 10x15-inch baking sheet as specified in the recipe, as Texas Sheet Cakes are supposed to be flat. You won't get a good result if you use a smaller baking pan in order to get a taller cake. Texas Sheet Cakes serve a large number of people, so it's a great choice for potluck parties.
1Preheat oven to 350°F (175°C). Grease and flour a 10x15 inch pan.
2Combine the flour, sugar, baking soda and salt. Beat in the sour cream and eggs. Set aside. Melt the butter on low in a saucepan, add the water and 5 tablespoons cocoa. Bring mixture to a boil then remove from heat. Allow to cool slightly, then stir cocoa mixture into the egg mixture, mixing until blended.
3Pour batter into prepared pan. Bake in the preheated oven for 20 minutes, or until a toothpick inserted into the center comes out clean.
4For the icing: In a large saucepan, combine the milk, 5 tablespoons cocoa and 1/2 cup butter. Bring to a boil, then remove from heat. Stir in the confectioners' sugar and vanilla, then fold in the nuts, mixing until blended. Spread frosting over warm cake.