Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13 inch pan. In a medium bowl, combine the buttermilk and baking soda. Set aside in a warm place.
In a large bowl, mix together 2 cups sugar, eggs, 3 tablespoons cocoa powder, oil and 2 teaspoons vanilla until blended. Stir in the boiling water, then gradually beat in the flour. Stir in the buttermilk mixture.
Pour batter into prepared pan. Bake in the preheated oven for 30 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool.
To make the Fudge Icing: In a saucepan, combine 1 cup sugar, 1/2 cup cocoa powder, milk, butter and 1 teaspoon vanilla extract. Bring to a boil, and cook for 1 minute. Remove from heat. Using an electric mixer, beat icing for 3 minutes, or until it thickens to spreading consistency. Pour over completely cooled cake.