Preheat oven to 350°F (175°C).

Grease a cookie sheet or two 9 x 12 inch cake pans.

In a large bowl, mix together flour, cocoa, salt, and baking soda.

In a separate bowl, combine sugar, melted margarine, buttermilk, vanilla, and beaten eggs.

Gradually add the dry ingredients to the wet ingredients, mixing with an electric mixer until well combined.

Pour the batter into the prepared cookie sheet or cake pans.

Bake for 35 to 40 minutes, or until a wooden skewer inserted into the center comes out clean.

Let cool before frosting.