Melt butter in the microwave in pulses of 30 seconds (1.5 minutes).
Add brown sugar, stir, and microwave again for 30 seconds. Stir again and microwave for another 1 minute. Cool for 10 minutes.
Pour the butter and sugar into a mixing bowl. Add eggs, egg yolk, vanilla extract, and salt and stir.
Add cocoa powder and flour and stir.
Pour the dough into a baking dish (8*8 inch) lined with parchment paper. Bake at 350F/175C for 25-30 minutes.
Remove the brownie from the oven and cool for 15 minutes.
Cut the Kitkat into separate sticks. Place the Kitkat sticks on the brownie and refrigerate for at least 1 hour.
Cut the cold brownie (then it will have smooth edges) and leave at room temperature to warm up a little before eating.