In a large bowl, sift together the cocoa powder, flour and baking powder. Add the salt. In a bowl of the electric mixer fitted with the paddle attachment, cream butter and sugar until light and fluffy, 5-8 minutes. Beat in eggs, one at a time, and add in vanilla. Reduce speed to low. Add half of the flour mixture, half of the sour cream, then the remaining flour mixture, remaining sour cream and the chopped chocolate. Do not over mix.