Preheat the oven to 325 degrees F, mist a 9x9-inch square baking pan*** with non-stick spray, and line with parchment. Place the cake flour, all-purpose flour, sugar, baking powder, baking soda, and salt in the bowl of an electric mixer, and stir on low speed to combine.
Add the butter, mixing on medium-low speed until the mixture resembles damp sand (about 1 minute).
Add the eggs, one at a time, scraping the bottom and sides of the bowl to be sure all of the ingredients are fully incorporated.
Add the sour cream, milk, and vanilla, mixing on medium speed for 2 to 3 minutes to aerate the batter and build the cake’s structure.
Fold in the chocolate chips, reserving a handful or two for the top of the cake.
Transfer the batter to the prepared pan, sprinkle the remaining chocolate chips on top, and bake until a toothpick inserted in the thickest part comes out clean or with a few moist crumbs (approx. 45 to 55 minutes).
Cool completely in the pan, then dust with powdered sugar.