Preheat your oven to 350°F (175°C). Grease a 9×13 inch baking pan or line it with parchment paper.

In a medium saucepan, melt the butter over low heat. Stir in both the granulated and brown sugars until well combined.

Remove the pan from heat and mix in the vanilla extract. After it cools slightly, whisk in the eggs one at a time, ensuring each is well incorporated.

In a separate bowl, combine the flour, cocoa powder, and salt. Slowly add this dry mixture to the wet ingredients, stirring until just combined.

Fold in the chopped Kit Kat bars evenly throughout the batter.

Pour the batter into the prepared baking pan and spread it evenly. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.

Let the brownies cool in the pan for a bit, then cool completely on a wire rack before slicing into squares.