Preheat oven to 350 F
Prepare a 8x8 inch or 9x9 inch cake pan by lining it with parchment paper and lightly grease it.
Zest the orange so that you end up with 2-3 tablespoons of orange zest and set aside. (While zesting, stop when you see the white part because it is bitter)
Cut up the orange (remove seeds if any) and puree the pulp. Keep it aside.
In a large bowl, sift the dry ingredients- all purpose flour, salt, baking powder, baking soda and cornstarch. Use a hand whisk to mix them well.
In a stand mixer fitted with a whisk attachment or hand held mixer, add eggs and sugar. Start whisking it on medium speed for about 3-5 minutes. The mixture will look light and fluffy. Make sure to scrape down the sides of the bowl at least once.
With the mixer still running, SLOWLY DRIZZLE oil.
Then add the orange zest, buttermilk, vanilla essence and the pureed orange pulp. Whisk for 30 seconds or until everything is well combined.
Remove the bowl from the stand mixer.
Add the dry ingredients into the wet batter and use a SPATULA to fold it. Keep folding gently until only a few streaks of dry flour can be seen. Do not overmix!
In a separate bowl, add 1 tablespoon of flour to the frozen cranberries and toss to coat them. This will prevent them from sinking to the bottom of the batter while baking.
Add 1 1/4 cup of the cranberries to the batter and gently mix.
Pour the batter into the prepared baking tin.
Scatter the remaining 1/4 cup cranberries on the top,
Place the baking pan in the preheated oven and bake for 35-40 minutes. Check for doneness by inserting a toothpick in the middle. It should come out clean or with a few moist crumbs. Give an additional 5 minutes if it is still very moist.
Allow to cool completely then remove from the baking tin and enjoy.