Put the margarine, sugar, eggs, flour, baking soda and cocoa in a large bowl, using an electric hand mixer beat together until just smooth. Using a metal spoon, fold in the sour cream. Spoon batter into paper liners.=
Bake the cupcakes in a preheated 350F oven for 20 minutes or until well risen and firm to the touch. Transfer to wire rack to cool.
To make frosting. Break the chocolate into a heat proof bowl. Set the bowl over a gently simmering double boiler and heat unill melted, stir occasionally. Remove from heat and cool slightly, then whisk in the sugar and sour cream until combined. Spread frosting over top of cupcakes and let set in refrigerator before serving. If liked, serve decorated with chocolate curls made by shaving semisweet chocolate with potato peeler.