Preheat the oven to 350F (180C or 170C fan forced). Line an 8x8 inch (20x20 cm) baking pan with baking paper, leaving on overhang around the edges. Alternatively, lightly grease the pan.
Add the butter chopped into cubes and finely chopped chocolate/chocolate chips to a large heatproof bowl. Melt using a double boiler over low heat. Or melt in the microwave in short 45-second intervals on medium power, and stir between each interval. Do not microwave on high power (which is often the default) as the chocolate can become grainy.
Whisk in the sugar, whisked eggs and vanilla extract.
Whisk in the flour, cocoa and salt. If the flour or cocoa are lumpy, sift them into the batter first.
Spoon about 1/2 of the batter into the prepared pan and smooth into an even layer.
Place a layer of Kit Kat wafers on top and lightly press down. If you want the pan entirely covered, you will need to trim some wafers. I fit 21 wafers, but I had to trim some.
Carefully drop spoonfuls of the remaining batter over the Kit Kats. Then spread the batter overtop to cover the wafers.
Chop/crush the remaining wafers and sprinkle over top of the brownie batter.
Bake in the preheated oven for 25-30 minutes, or until an inserted toothpick comes out clean or with a few damp crumbs. Bake time will vary depending on ovens and how gooey you like your brownies.
Cool the brownies completely before slicing - this will take about 3-4 hours. If you lined the pan with parchment, lift the cooled brownies out of the pan and place on a cutting board. Always slice brownies with a sharp knife and not a table knife.