Preheat the oven to 350°F (180°C) conventional (see note 1 if using a convection oven with a fan) and grease two 8-inch cake tins (I use my homemade cake release). Line the bottoms with parchment paper, as the pecans can sometimes stick to the bottom of the tins. Set aside. In a medium bowl, sift together the flour, cornstarch, baking powder, and salt. Using a whisk, mix until well combined. Set aside.
In a large bowl, add the butter, vegetable oil, granulated sugar, and brown sugar. Using an electric mixer, cream together on medium speed for 2 minutes until light and creamy.
Add in the eggs one by one, mixing on low speed for about 10–15 seconds between each addition, or until well combined.
Add in the sour cream and vanilla and mix on low speed until everything is well combined. Now set your mixer aside, as the remainder of the batter will be finished by hand.
Add half of the dry ingredients to the wet mixture and gently fold with a spatula until just combined. Then add in the milk and fold until just combined. Lastly, add in the remaining dry ingredients and gently fold until you can no longer see any streaks of flour. Be careful not to overmix, as this can make the cake dense instead of soft and tender.
Add in the cooled toasted pecans, including any leftover buttery sugar mixture, and gently fold until evenly distributed throughout the batter. Do not overmix.
Divide the batter evenly between the two prepared 8-inch cake tins. I like to weigh my cake tins to make sure there is an even amount of batter in each. Smooth out the tops and lightly drop the tins on the counter to remove any large air bubbles.
Bake for 37 minutes, or until a toothpick inserted into the center comes out clean or with just a few moist crumbs attached.
Once baked, allow the cake layers to cool in the tins for 20 minutes. Carefully run a thin knife around the edges to release the cakes, then turn them out onto a wire rack to cool completely before frosting.