Preheat the oven to 350°F (180°C), spray a 9x13 baking dish with non-stick spray, and line it with parchment paper so that there is an overhang on either side. Set aside.
Sift the all-purpose flour and cocoa powder into a mixing bowl, add the salt, and mix together. Set aside.
Add the butter and ½ cup chocolate chips or chopped chocolate to a microwave-safe bowl and microwave in 30 second intervals, bringing it out to stir every 30 seconds until melted.
Add the sugar, eggs, and vanilla to the melted butter and chocolate and whisk together until well mixed.
Add the wet ingredients to the dry and mix into a thick batter.
Transfer half the batter to the prepared 9x13 dish and smooth down.
Add the whole KitKat fingers on top, then add the remaining brownie batter, spoon it over the KitKats, and spread it out evenly. Sprinkle the top with the remaining crushed KitKats.
Place into the oven and bake for 30-35 minutes. You will know the brownies are done when they are dry and shiny on top and when a toothpick inserted into the center comes out with melted chocolate or moist crumbs but NO wet batter.
Let the brownies cool and firm up in the pan before lifting them out with the parchment paper overhang. Let them cool completely before cutting them into squares.