1. Prepare Oven and Pan:

• Preheat the oven to 375°F (190°C).
• Grease and flour a 10x15x2 inch baking pan (jelly roll pan with sides) very well. Alternatively, spray generously with non-stick baking spray that includes flour. Set aside.

2. Start the Cake Batter (Boiled Mixture):

• In a large saucepan, combine the 1 cup margarine (or butter), 1 cup water, and 4 Tablespoons cocoa powder.
• Place the saucepan over medium-high heat and bring the mixture to a rolling boil, stirring occasionally. Once boiling, remove it from the heat.

3. Mix Remaining Cake Ingredients:

• While the cocoa mixture heats, in a separate large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking soda, and salt.  
• In a smaller bowl, whisk together the room temperature sour cream, beaten room temperature eggs, and vanilla extract (if adding to cake).

4. Combine Cake Batter:

• Pour the hot boiled cocoa mixture carefully into the large bowl containing the dry ingredients (flour, sugar, etc.). Whisk together until just combined.
• Add the sour cream/egg mixture to the bowl. Whisk again until everything is smooth and well combined. Do not overmix.

5. Bake the Cake:

• Pour the relatively thin batter into the prepared 10×15 inch baking pan. Spread it evenly.
• Bake for approximately 20 minutes (sheet cakes bake quickly!), or until a toothpick inserted into the center comes out clean. Keep an eye on it as bake times vary.  

6. Make the Frosting (While Cake Bakes/Cools Slightly):

• While the cake is baking or just as it comes out, prepare the frosting. In a medium saucepan, combine the ½ cup margarine (or butter), 6 Tablespoons milk, and 4 Tablespoons cocoa powder.
• Place the saucepan over medium heat and bring the mixture just to a boil, stirring constantly.
• Once it boils, remove it immediately from the heat.
• Off the heat, stir in the 1 teaspoon vanilla extract.
• Gradually stir or whisk in the sifted powdered sugar until the frosting is smooth.
• Stir in the chopped walnuts or pecans.

7. Frost the Warm Cake:

• As soon as the frosting is mixed (while it’s still warm and pourable) and the cake is just out of the oven (still warm), pour the warm frosting evenly over the surface of the warm cake in the pan.
• Quickly spread the frosting out to cover the entire cake surface using an offset spatula or the back of a spoon. Work fast as the frosting starts to set as it cools.  

8. Cool and Serve:

• Let the Texas Sheet Cake cool completely in the pan on a wire rack before slicing into squares. The frosting will set to a slightly fudgy consistency.
• Slice and serve directly from the pan. Enjoy this classic treat!