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