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 eggs, melted butter, and vanilla extract until smooth.
Evenly divide half of the batter into the paper liners. Place one kitkat candy bar half into each brownie cup and cover with more brownie batter. Bake for 15-20 minutes.