Preheat oven to 350. Grease a 13 x 9 inch baking pan. Set aside.
For topping, in bowl, stir together the 1/2 cups flour, brown sugar and cocoa powder. using a pastry blender, cut in the 1/4 cup butter til crumbly. Stir in chocolate pieces and pecans. Set aside
In bowl, stir together the 3 3/4 cups flour, the baking pow2der, baking soda, and salt. Sprinkle with orange peel. using your fingers, distribute orange peel evenly through flour mixture.
In extra large mixing bowl, beat 3/4 cup butter and the sugar with a mixer on medium pseed for 2 minutes. Add eggs, one at a time, beating well after each addition. Beat in sour cream, milk, orange juice and vanilla til mixed. Gradually beat in flour mixture on low speed. Beat about 2 minutes more or til smooth.
Pour half of the batter into prepared pan, spreading evenly. Sprinkle batter with half of the topping. using the back of a spoon, press topping into batter. Spread remaining batter in pan. Sprinkle batter with remaining topping.
Bake for 50 to 55 minutes or til wooden toothpick inserted near center comes out clean. Cool in pan on wire rack about 15 minutes to serve warm, or cool completely. serves 24