Preheat the oven to 350 F. Line a 12 cup muffin pan with paper liners. In a large bowl, whisk together flour, sugar, cocoa powder, salt, and baking soda. Add in the eggs, melted butter, and vanilla extract, and mix until smooth.
Evenly divide the brownie batter between the prepared muffin pan. In a large bowl, combine cake mix with eggs and vegetable oil until smooth.
Evenly divide the cookie dough over the brownie batter. Bake for 20-25 minutes.