Preheat oven to 350 degrees. Spray an 8 by 4-inch baking dish with cooking spray. In a mixing bowl whisk together flour, poppy seeds, baking powder, baking soda and salt, set aside. Add sugar and orange zest to the the bowl of an electric stand mixer fitted with the paddle attachment. Mix the sugar and zest together for a minute or so until the mixture becomes moist and pale orange. Add butter to sugar mixture and whip on medium speed until pale and fluffy. Mix in eggs one at a time, then add vanilla until blended. Whisk together buttermilk and sour cream and orange juice. Working in two batches, beginning and ending with flour mixture, alternate with the milk mixture, mixing just until combined after each addition. Transfer batter to prepared loaf pan and bake for about 40-45 minutes, tenting with foil at 40 minutes until toothpick inserted into center comes out with few crumbs. Remove from oven and allow to cool 5 minutes in loaf pan, then invert loaf onto a wire rack. Combine orange juice and powdered sugar and whisk together so there are no lumps. Drizzle glaze over cooked loaf and sprinkle with extra poppyseeds as desired. Slice and enjoy!