Heat your oven to 350. Line a 10x15-inch baking sheet with foil, with the short ends folded over the sides, and spray the foil with non-stick spray.
Combine all of the dry ingredients in the bowl of a stand mixer. Add the wet ingredients to the bowl and mix on low until everything is combined. Scrape the sides and bottom of the bowl and mix on medium for 2 minutes.
Transfer the batter to the prepared baking sheet and spread it evenly. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean. Cool completely before frosting.
While the bars are cooling, prepare the cream cheese frosting: In the bowl of a stand mixer, cream together the cream cheese and butter. Add 4 cups of the powdered sugar, and mix on low until it's mostly combined. Add the vanilla and 2 teaspoons of the milk. Mix on low for 60 seconds; scrape the sides and bottom of the bowl. Mix on medium-low until the frosting is very smooth and creamy. If it seems too wet add a little more powdered sugar; if it seems like it is too thick and will be hard to spread, add another teaspoon of milk. Be careful, though; cream cheese frosting is gooey by nature, so it can be tricky to know when you have the right consistency. If in doubt, try spreading some on the bars.