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 melted butter, eggs, and vanilla extract and mix until smooth.
Place one Oreo cookie into each prepared muffin cup. Pour the brownie batter over the Oreos. Bake for 15-20 minutes.