1. Preheat the oven to 180°C, and line a cake loaf pan with foil. Lightly coat the foil with nonstick cooking spray.
2. To prepare the cake, whisk together the flour, baking powder, baking soda, salt, and orange zest in a medium bowl.
3. In a separate bowl, whisk together the butter, egg whites and vanilla extract. Stir in the Greek yogurt, mixing until no large lumps remain.
4. Into this same bowl, stir in the orange juice and 2 tablespoons of milk. Alternate between adding the flour mixture and remaining milk, beginning and ending with the flour mixture, and stirring just until incorporated. (For best results, add the flour mixture in 3 equal parts.) Gently fold in the cranberries.
5. Spread the batter into the prepared pan. Bake at 350°F for 50-60 minutes or until the top feels firm to the touch and a toothpick inserted into the centre comes out clean.
6. Cool in the pan for 10 minutes before lifting out the cake by the foil and transferring it to a wire rack to cool completely.