Preheat oven to 350 degrees F. Grease and parchment line a standard loaf pan (9 x 5 inch) with overhangs.
In a bowl, mix together the flour, baking powder and salt until combined and set aside.
In a stand mixer with the paddle attachment (or with a handheld mixer), combine the sugar and orange zest and mix on low for one minute.
Increase the speed to medium low and add the eggs one at a time until fully combined, then add in the oil and vanilla.
Mix to combine. Alternating dry and wet ingredients with the mixer on low, add in half of the flour mixture, all of the sour cream, then the rest of flour mixture just until it comes together. Add in the orange juice and poppy seeds and mix until just combined.
Pour the batter into the prepared loaf pan and bake for 45-55 minutes. When done, the cake will start pulling away from the sides of the pan and a toothpick comes out clean from the center. Remove the loaf and transfer to a cooling rack.
Immediately prepare the simple syrup by bringing all the sugar, orange juice, and water to a boil, then remove from the heat and brush (or carefully spoon) all over the loaf. You will use all of it.
Once the loaf is completely cool, make the glaze by combining the powdered sugar, orange zest, and orange juice in a bowl and mix with a whisk until smooth. Remove the loaf from the pan and pour the glaze over the top. Sprinkle with additional orange zest. Slice and enjoy!