Preheat the oven to 350 degrees F. Line the bottom of two, 12-cup muffin tins with paper liners. Melt the chocolate in a double boiler over simmering water. In a bowl, stir baking soda into the boiling water. Whisk this mixture into chocolate, and then set aside to cool slightly.
Beat butter and sugar until light and pale yellow in colour. Beat in the eggs one at a time and continue beating until mixture is light and thick. Add the flour and buttermilk,
alternating between a 1/3 of each, and blend well. Mix in the melted chocolate and vanilla. Pour the batter into the paper cups. Bake 25 minutes, or until a cake tester comes
out clean when inserted into the centre of a cupcake. Allow cupcakes to cool briefly in the muffin tin before removing and placing on a wire rack to cool completely.
Make the icing by beating together the 1/2 lb. butter and icing sugar until light and well combined. (Make chocolate icing, if desired, by adding cocoa powder to taste to some of the icing.) Top cupcakes with icing; decorate with a Halloween candy.