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