Preheat your oven to 350°F (175°C) and line the baking sheets with parchment paper.

In a mixing bowl, cream together the unsalted butter, brown sugar, and granulated sugar until light and fluffy, about 2-3 minutes.

Add one whole egg and one egg yolk, mixing well after each addition. Then add the vanilla extract and mix until combined.

Whisk together flour, cocoa powder, baking soda, and salt in a separate bowl, then gradually add to wet ingredients, mixing gently until combined.

Fold in chocolate chips and chopped Kit Kat bars until evenly distributed throughout the dough.

Scoop dollops of dough onto prepared baking sheets, leaving enough space for spreading. Bake for 10-12 minutes or until edges are set.

Let cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.