Grease and flour two 9-inch cake pans or a 9 x 13-inch baking pan.  Set aside.  Sift together flour, soda and salt.  Set aside.  In a large mixing bowl cream butter, cocoa and sugar for approx. 3 minutes.  Add eggs and vanilla and beat until smooth and creamy.  Add sour cream and mix until smooth.  Add flour mixture and mix just until blended.  Slowly add boiling water and beat until smooth.  Bake in a preheated 350 degree oven for 25-30 minutes or until cake springs back when lightly touched.  Cool in pans for 10 minutes.  Invert to a cooling rack to cool.  Bake cupcakes for 18-20 minutes.

In a mixing bowl, blend butter.  Add cocoa, 3 cups of powdered sugar, salt and 4 tablespoons of milk.  Mix until smooth.  Scrape down the sides of the bowl and add vanilla.  At this point check for consistency.  If you would like your frosting thicker add the extra powdered sugar.  If it needs to be thinner add more milk. Spread over cooled cake.  Sprinkle with chopped walnuts.