Preheat your oven to 350 degrees Fahrenheit (175 degrees Celsius).Grease and flour a 9x13 inch pan.Sift together the flour, cocoa, baking soda and salt in a medium bowl and set it aside.
In a large bowl, cream the margarine and sugar until it becomes light and fluffy.Beat in the eggs one at a time, then stir in the vanilla.Add the flour mixture to the large bowl alternately with the milk, mixing just until incorporated.
Bake in the preheated oven for 40 to 45 minutes, or until a toothpick inserted into the center of the cake comes out clean.Allow the cake to cool completely before serving.