Preheat the oven to 350°F and line an 8x8-inch baking pan with parchment paper, leaving some overhang for easy removal.
In a medium bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt until combined, then set aside.
In a large mixing bowl, beat the softened butter, vegetable oil, and sugar together for 3 to 4 minutes until light and fluffy.
Add the eggs one at a time, beating well after each addition, then mix in the vanilla extract.
Add the milk and sour cream and mix until combined.
Add the dry ingredients and milk mixture alternately, beginning and ending with the dry ingredients, mixing only until the batter comes together.
Toss the mini chocolate chips with 1 tablespoon of flour, then gently fold them into the batter.
Spread the batter evenly into the prepared pan.
Bake for 38 to 45 minutes, or until the center reaches about 205°F or a toothpick inserted into the center comes out mostly clean with a few moist crumbs.
Allow the cake to cool in the pan for at least 30 minutes before slicing and serving