In a medium bowl mix the flour and baking powder.
In a large mixing bowl, rub the orange zest with the sugar.
Mix in the eggs, using an electric mixer, or a whisk. (or a standing mixer fitted with the paddle attachment).
Add the oil, melted butter, juice, sour cream and vanilla extract and mix until smooth.
Stir in the dry ingredients.
Remove the pan from the refrigerator and pour the batter over the oranges.
Bake for 45-55 minutes at an 350F preheated oven until the cake is golden brown and a tooth pick comes out from the center of the cake with a few moist crumbs.
Remove from the oven and allow the cake to cool in the pan for about 20-25 minutes.
Invert the cake over a serving dish.