In a large metal bowl set over a pan of barely simmering water melt the chocolate and the butter,stirring until the mixture is smooth, stir in the sour cream and the vanilla, and let the mixture cool.In a bowl beat together the eggs and the sugar until the mixture is thick and pale and forms a ribbon when the beaters are lifted and beat in the cream cheese. Stir in the chocolate mixture and fold in the pecans. Pour the filling into the prepared crust and bake the cheesecake in the middle of a preheated 325 degree F oven for 2 hours, or until it is just set. (The cake will fall in the middle.) Let the cheesecake cool in the pan on a rack, chill it, covered loosely, overnight, and remove the side of the pan. Sprinkle the confectioners' sugar and the cocoa powder decoratively over the cheesecake.