Start by gathering all the necessary ingredients. Preheat your oven to 350°F (175°C), and prepare a 10x15-inch jelly roll pan by greasing and flouring it.

For the cake: In a large mixing bowl, combine the flour, sugar, baking soda, and salt. Add the sour cream and eggs, mixing well until the batter is smooth.

In a small saucepan, melt the butter over low heat. Gradually stir in the water and cocoa powder, and bring the mixture to a gentle boil. Once it reaches a boil, remove it from the heat and allow it to cool slightly.

Once cooled, blend the cocoa mixture into the flour and egg mixture until thoroughly combined. Pour the prepared batter into the jelly roll pan, spreading it evenly.

Bake the cake in the preheated oven for approximately 20 minutes, or until a toothpick inserted into the center comes out clean.

While the cake is baking, prepare the icing: In a large saucepan, combine the butter, milk, and cocoa powder. Heat the mixture over medium heat until it reaches a boil. Remove from the heat immediately.

Stir in the confectioners' sugar and vanilla extract until the icing is smooth and creamy. If desired, fold in the chopped walnuts for added texture.

Once the cake is out of the oven and still warm, spread the icing evenly over the top.

Serve warm and enjoy this delicious Texas Sheet Cake!