Preheat the oven to 350 F. In a large bowl, whisk flour, sugar, cocoa powder, salt, and baking soda together. Add in peanut butter, eggs, melted butter and vanilla extract and whisk until smooth.
Spread half of the batter into a greased 8-inch square baking dish. Top with candy bars. Cover with the remaining brownie batter. Bake for 25-30 minutes.