Preheat the Oven: Set your oven to 350°F (325°F if using a convection oven).
Prep the Pan: Grease a 12cup bundt pan with 12 tbsp of softened butter, ensuring it's wellcoated. Dust with flour, making sure it covers all areas, especially if your pan has intricate designs.
Make Orange Sugar: Mix the orange zest with the granulated sugar, rubbing it between your fingers to release the oils.
Combine Dry Ingredients: In a medium bowl, whisk together the flour, cornstarch, salt, baking powder, and baking soda.
Beat the Butter and Sugar: In a large bowl, beat the butter with an electric mixer on medium speed for 2 minutes. Add the orange sugar and beat on mediumhigh until fluffy and pale, about 2 more minutes.
Add Eggs and Wet Ingredients: Add eggs one at a time, beating well after each. Mix in the sour cream and orange juice on low speed.
Combine Mixtures: Gradually add half the dry ingredients, then the milk, and finally the remaining dry ingredients, mixing on low speed until just combined. Finish with a spatula to avoid overmixing.
Bake: Pour the batter into the prepared bundt pan and spread it evenly. Bake for 5565 minutes, covering with foil if it browns too quickly. It's done when a toothpick inserted comes out clean.