A recipe for chocolate sheet cake can be found on the Taste of Home website and calls for sugar, eggs, cocoa, butter and flour along with ingredients for chocolate frosting. Another good recipe is Texas sheet cake from MyRecipes.com.
To make a chocolate sheet cake from scratch, stir together 2 cups of sugar, 2 cups of flour, 1 teaspoon of baking soda and 1/2 teaspoon of salt in one bowl and 1/2 cup of buttermilk, 2 eggs and 1 teaspoon of vanilla extract in another bowl. In a saucepan, boil 1 cup of water mixed with 1/2 cup of butter and 1/4 cup of baking cocoa. Remove the chocolate sauce from the heat, and pour it and the buttermilk mixture into the flour mixture. Pour the batter into a baking pan, and cook at 375 degrees Fahrenheit for 20 to 22 minutes.
While the cake cooks, make the icing by adding 1/2 cup of butter, 1/4 cup of baking cocoa and 1/3 cup of milk to the saucepan, and bringing it to a boil. Stir often, and once everything is well-blended, remove the saucepan from the heat. Mix in 2 cups of confectioners' sugar and 1 teaspoon of vanilla extract. Frost the cake while it is still hot, and top with 1 cup of chopped walnuts. Cool before serving.