Preheat the oven to 350 F. 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.
Spread half of the batter into a greased 8-inch square baking dish. Top with kitkat candy bars. Cover with the remaining brownie batter. Bake for 20-25 minutes.