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 until smooth.
Fold in the butterfinger candy bars. Evenly divide the brownie batter between the prepared muffin pan. Bake for 15-20 minutes.