Preheat the oven to 350˚F. Line an 8x8-inch baking pan with parchment paper.
To make the brownies, combine the Brownie Mix, water and oil in a large bowl. Stir until well mixed.
Spread the mixture into the prepared pan, smoothing evenly. Bake for 10 minutes.
While the brownies are baking, prepare the chocolate chip cookie mixture. Combine the oil, brown sugar, sugar, milk, and vanilla extract in a large bowl or the bowl of a stand mixer. Beat on medium speed until the mixture is well-combined and emulsified.
Combine the All-Purpose Flour Mix, xanthan gum, baking soda, salt, and baking powder in a medium bowl. Whisk to combine. Beat on low speed and add the flour mixture. Beat until the mixture is combined and sticks together. Stir in the Mega Chunks.
After the brownies have baked for 15 minutes, remove from the oven. Dollop the chocolate chip cookie mixture onto the par-baked brownies, pressing down into the batter.
Bake for an additional 28-33 minutes, until a toothpick inserted in the brownies comes out clean, and the chocolate chip cookie portions are golden brown. Let cool in the pan for at least 1 hour.
Serve, storing leftovers in an airtight container in the refrigerator for up to 3 days, and any other leftovers to the freezer.