Preheat your oven to 350°F and line an 8x8 baking dish with parchment paper.
Combine the butter cubes and chocolate chips in a microwave-safe bowl. Melt in 30-second intervals, stirring until smooth.
Add sugar, eggs, and vanilla extract to the melted chocolate mixture. Whisk until combined.
Add the dry ingredients: Gradually whisk in the flour, cocoa powder, and salt.
Pour half of the batter into the prepared baking dish. Place a layer of Kit Kat bars on top and press gently.
Pour the remaining batter over the Kit Kat layer. Chop up the remaining Kit Kats and sprinkle them over the batter.
Bake for 25-30 minutes, or until a toothpick inserted into the center comes out mostly clean (except for a few crumbs from the Kit Kats). Let the brownies cool completely before slicing and serving.