Preheat oven to 350 degrees and line an 8x8 baking pan with parchment paper, then spray with cooking spray.
Add flour, sugars, cocoa powder, and salt to a medium sized mixing bowl, whisking to combine.
½ cup flour, ¼ cup granulated sugar, 1 cup light brown sugar, ¾ cup cocoa powder, ¼ teaspoon salt
In a separate large bowl, add eggs and egg yolk, whisking for 1-2 minutes until frothy. Add slightly cooled melted butter, vanilla extract, and vegetable oil, whisking to combine.
2 eggs, 1 egg yolk, 1 stick butter, 2 teaspoons vanilla extract, ¼ cup vegetable oil
Gradually add dry ingredients to wet ingredient mixture, stirring to combine with a wooden spoon. Mix in chocolate chips. If the batter seems very thick, add in 1 tablespoon of warm water and mix until glossy.
½ cup semisweet chocolate chips, ½ cup bittersweet chocolate chips, 1 tablespoon water
Pour batter into prepared pan, smooth the top with a spatula, and bake for 30-35 minutes until a toothpick just comes out clean. Do not overbake.
Remove from oven and let cool at room temperature for at least 1 hour in the pan.
While the brownies are cooling, make the cream cheese frosting. Add the butter and cream cheese to the bowl of a stand mixer fitted with the paddle attachment, or to a large bowl if using a hand mixer. Beat on high for 1-2 minutes, scrape down sides and bottom of bowl, then continue beating for 1-2 additional minutes or until no lumps remain. Add in the powdered sugar, milk (I recommend starting with 1 tablespoon and adding more if needed), and vanilla, mix on low speed until combined, then increase speed to high and beat for 2-3 minutes until pale and fluffy.
¼ cup butter, 5 ounces cream cheese, 3 ½ cups powdered sugar, 1-2 tablespoons milk, ½ teaspoon vanilla extract
Once brownies are cooled, remove them from the pan, spread frosting over the top, then sprinkle with chocolate shavings if desired. Use a sharp knife to cut the brownies into either 9 or 16 squares, and enjoy!
chocolate shavings