Preheat the oven to 180C/350F/Gas 4. Line an 8x8-inch square baking pan with foil or parchment paper, and set aside.
Place the butter, sugar, cocoa powder, and salt into a microwave-safe bowl, and heat for 1 minute. Stir well, then microwave for another minute. The mixture should very warm to the touch.
Add the eggs and vanilla, and mix well until combined. Fold in the flour until fully combined, then gently fold in the chocolate chips. Be sure not to let them ALL melt into the warm batter.
Spoon half of the batter into the prepared pan. Place the whole KitKat bars on top, then spoon the remaining batter over the KitKats, spreading out evenly. Sprinkle with the crushed KitKat bars.
Bake for 35 - 40 minutes, or until the centre is no longer jiggly and a toothpick inserted comes out almost clean.
Allow to cool completely in the pan on a wire rack before cutting into squares.
Video
Notes
Brownies stay fresh, stored in an airtight container, at room temperature or in the fridge for up to 1 week. They also freeze well for up to 3 months. Thaw overnight in the fridge before serving.