Texas Sheet Cake

 Preheat the oven to 350 degrees. Grease and lightly flour a jelly roll pan (12 x 17 – 1 inch deep). 
 In a medium saucepan over medium-low heat, melt 1 cup butter, add water, and 4 tablespoons cocoa. Bring the mixture to a low boil; whisking frequently. Remove from the heat; cool for 20 minutes. 
 In a large bowl, combine flour, sugar, baking soda, and salt. In a separate small bowl, whisk together the sour cream, eggs, and vanilla extract. 
 Pour the egg and sour cream mixture and the chocolate mixture into the flour mixture. Stir to combine, mixing just until incorporated. Pour the batter into the prepared pan and bake for 18-22 minutes or until a toothpick inserted in the center comes out clean. 
 For the frosting, combine milk, 1/2 cup butter, and 4 tablespoons cocoa powder in a large saucepan over medium-low heat. Bring to a low boil whisking constantly. Remove from the heat. Working quickly, stir in the powdered sugar and the pecans or walnuts. Pour the icing over the warm cake, spreading with a spatula. Let the frosting set for about 15 minutes before slicing and enjoying. 

Notes :

 Use a jelly roll pan for the best results, as its dimensions are ideal for the thickness of this cake. 
 For the frosting, be prepared to work quickly to spread it while the cake is still warm for the best texture. 
 This cake can be stored at room temperature and covered tightly for several days.