Preheat oven to 180°C (350°F) conventional (see note 1 if using a convection oven with a fan) and grease a 9-inch springform cake tin (I use my homemade cake release). Line the bottom with parchment paper as well. Prepare the orange: Zest the entire orange into a large bowl and set aside for later (see note 2). Using a small knife, remove and discard the white pith. You should be left with the flesh of the orange. Slice it in half, remove any seeds if necessary, and blend until smooth. Set the blended orange aside for later (see note 3).
Sift together flour, cornstarch, baking powder, and salt. Using a whisk, mix until well combined, then set aside.
Return to the large bowl with the orange zest from earlier. Add in the butter, oil, and sugar. Using a hand or stand mixer on medium speed, cream for 2 minutes until light and fluffy.
Next, add in the eggs one at a time while mixing on a low speed, making sure to mix well between each addition.
Next, add in the blended orange from earlier, along with the milk and vanilla. Mix on low speed until well combined. The batter may look slightly curdled — but that’s okay! Remove any batter from the attachments, as the rest of the mixing will be done by hand.
Add the pre-sifted dry ingredients to the wet ingredients, and using a spatula, gently fold until just combined and the batter is uniform. Be careful not to over-mix.
Pour the batter into the prepared cake pan. Smooth out the top, then gently tap the pan on the counter once to release any large air bubbles. Bake for 35 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Allow the cake to cool in the pan for 15 minutes, then run a thin knife around the edges to loosen it. Carefully remove the outer ring of the springform pan. Let the cake cool to room temperature before transferring it to a serving plate or cake stand. Finish with a light dusting of powdered sugar, slice, and enjoy!