In a mixing bowl, add softened butter and both white and brown sugars. Beat together until all of the butter is mixed and there are no clumps remaining.
Beat in eggs one at a time and add vanilla extract. Remove Kit Kats and the chocolate bar from their packaging and chop into small pieces.
Using a sifter, sift cocoa powder and flour into the mixture. Add the remaining dry ingredients: salt, cornstarch, and baking powder to the batter. Use a hand mixer to combine all ingredients. Note: batter will be sticky and thick. That’s ok!!
Toss in all of the chopped Kit Kats and chocolate and stir. It’s going to be quite thick.
Preheat the oven to 350°F. Line or grease a baking sheet. Using a greased cookie scooper (just spray the scoop with cooking spray), scoop cookies into balls and place on the cookie sheet (this recipe makes ~24 cookies so you’ll need two trays or to bake in two batches).
Bake for 8-10 minutes or until the edges of the cookies are golden brown. Remove promptly from the baking sheet and allow them to cool on a wire rack. While they’re still warm, push in a few extra Kit Kat pieces into the tops if desired. Enjoy!