Preheat oven to 350°F. Lightly grease a 13x9 inch baking dish. Set aside.
In a medium size bowl whisk the flour, baking powder, baking soda, and salt. Set aside.
In the bowl of a stand mixer fitted with paddle cream the butter and sugars together until light and fluffy, about 2-3 minutes on medium speed.
Add the eggs, one at a time, beating well after each addition. Mix in vanilla and milk.
With mixer on low add half of the dry ingredients, then add the sour cream, and finish by adding the remaining dry ingredients. Mix until just combined. Fold in chocolate chips.
Scoop the batter into the prepared baking dish. Use a spatula to spread the batter into an even layer. Sprinkle the topping over the batter and then top with the additional ½ cup of chocolate chips.
Bake for 35-40 minutes or until a toothpick inserted in the center comes out with a few moist crumbs, but no wet batter. (Mine baked for 38 minutes).