In a 3-4 quart pan over low heat, stir butter with cocoa until melted. Off the heat, beat in sugar. Add eggs and vanilla; beat well. Add flour, pecans, and chocolate; mix well. Spread batter in buttered 9x13" pan. Bake in a 325~ oven until top feels dry in center, about 35 minutes. Cool in pan. If baking ahead, wrap airtight when cool and hold up to a day. Cut into 12 to 16 pieces.