Texas Sheet Cake

It had been so long since I made this cake that I was actually wondering if it was as good as I remembered.  I made it yesterday and let me tell you, it is every bit as good as I remembered.  Takes a little bit of work, but the results are well worth it!
Cake:

• 2  cups flour
• 2 cups white sugar
• 1 teaspoon baking soda
• 1/2 teaspoon salt
• 1/2 cup sour cream ( I didn’t have any sour cream so I added 2 teaspoons of lemon juice to milk to equal 1/2 cup, also you could use white vinegar)
• 2 eggs
• 1 cup butter
• 1 cup water
• 5 tablespoons unsweetened cocoa powder

Icing:

• 6 tablespoons milk
• 5 tablespoons unsweetened cocoa powder
• 4 cups confectioner’s (powdered) sugar
• 1 teaspoon vanilla extract (I am really picky about this and only use pure vanilla extract)
• 1 cup chopped walnuts

1. Preheat oven to 350 degrees.  Grease and flour a 10X15 inch pan.
2. Combine the flour, sugar, baking soda and salt in a large bowl.  Beat in the sour cream and eggs and set aside.
3. Melt the 1 cup butter in a saucepan over low heat.  Add the 1 cup of water and the 5 tablespoons cocoa.  Bring mixture to a boil and immediately remove from heat.  Allow to cool slightly, then stir mixture into the first mixture, mixing till well blended.
4. Pour batter into prepared pan.  Bake in the preheated oven for 20 minutes or until a toothpick inserted into the center come out clean.
5. For 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 confectioner’s sugar and vanilla, then fold in the nuts, mixing until blended.  Spread frosting over warm cake.