Preheat your oven to 350°F (175°C). Grease your 9x13 inch baking pan to prevent sticking.
In a large mixing bowl, whisk together the sugar, flour, salt, and cocoa powder until well combined. Set aside.
In a medium saucepan, melt the butter over medium heat. Add the water and bring it to a boil. Once boiling, pour the butter and water mixture into the dry ingredients and mix until combined.
Stir in the buttermilk, eggs, and baking soda into the mixture until smooth and well incorporated.
Pour the batter into the prepared baking pan and spread it evenly. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Once baked, remove the cake from the oven and let it cool in the pan for about 10 minutes. Then, transfer it to a cooling rack to cool completely.
In a separate bowl, beat together the ½ cup of butter, cocoa powder, powdered sugar, milk, and vanilla extract until smooth and creamy.
Once the cake is completely cooled, spread the frosting evenly over the top.
Chop the cake into bite-sized squares or rectangles. Sprinkle chopped walnuts on top for an added crunch and flavor.